add missing info to linked prefab doc

- missing doc for new way to create linked prefab + small fixes

Author: vkovec

Packages/com.unity.formats.fbx/Documentation~/prefabs.md

@@ -4,12 +4,16 @@ A Linked Prefab is a Prefab which maintains a link to an FBX file and is respons
 
 During an update, the Linked Prefab preserves components and object references to the greatest extent possible. If you delete a node in the FBX, the Linked Prefab deletes the corresponding object in Unity along with its components.
 
-The Linked Prefab contains a FbxPrefab script component that is responsible for recording changes and merging FBX Model changes into the Linked Prefab. The automatic update happens whenever the Asset database reimports the corresponding FBX Model, after all Assets have been imported.
+The Linked Prefab contains an FbxPrefab script component that is responsible for recording changes and merging FBX Model changes into the Linked Prefab. The automatic update happens whenever the Asset database reimports the corresponding FBX Model, after all Assets have been imported.
 
 Use __Convert To Linked Prefab Instance__ (right-click on an object in the hierarchy view, or menu: __GameObject__ > __Convert To Linked Prefab Instance__) to replace the GameObject hierarchy with an instance of a Prefab that is linked to an FBX Model. 
 
 __Convert To Linked Prefab Instance__ exports each selected GameObject hierarchy and writes both a .prefab and a .fbx file. Linked Prefabs use the same rules as for exporting: all selected objects and their descendants are exported to a single FBX file. If you select both a parent and a descendant, the Linked Prefab only exports the parent’s hierarchy.
 
+NOTE: if the selected object is already linked to an existing FBX Model, then the hierarchy will not be re-exported, instead the Linked Prefab will link to the already existing FBX file.
+
+Use __Convert To Linked Prefab__ (right-click on an FBX or prefab in the project view, or menu: __Assets__ > __Convert To Linked Prefab...__) to generate the Linked Prefab from the selected file without modifying the scene.
+If an FBX file is selected, then a Linked Prefab will be created linking to the selected FBX. If a prefab is selected, then the prefab will be exported to an FBX file and then transformed into a Linked Prefab.
 
 <a name="ConvertOptions"></a>
 ## Convert Options window
@@ -27,14 +31,14 @@ When converting to a Linked Prefab, the following window opens, displaying optio
 | __Export Path__           | Specify the location where the FBX Exporter will save the FBX file. |
 | __Prefab Name__           | Specify the name of the Linked Prefab's file.                |
 | __Prefab Path__           | Specify the location where the FBX Exporter will save the Linked Prefab file. |
-| __Source__                | Transfer the transform animation from this object to the __Destination__ transform. __Destination__.<br/><br/>**Notes:** - __Source__ must be an ancestor of __Destination__.<br/> - __Source__ may be an ancestor of the selected object. |
-| __Destination__           | Which object to transfer the transform animation to.<br/><br/>This object receives the transform animation on objects between Source and Destination as well as the animation on the Source itself. |
+| __Source__                | Transfer the transform animation from this object to the __Destination__ transform.<br/><br/>**Notes:**<br/> - __Source__ must be an ancestor of __Destination__.<br/> - __Source__ may be an ancestor of the selected object. |
+| __Destination__           | Which object to transfer the transform animation to.<br/><br/>This object receives the transform animation on objects between __Source__ and __Destination__ as well as the animation on the __Source__ itself. |
 | __Export Format__         | Select the format for the FBX Exporter to use when exporting the FBX file (ASCII or binary). |
 | __Include__               | __Convert to Linked Prefab Instance__ always exports both Models and Animation in the hierarchy. |
 | __LOD level__             | __Convert to Linked Prefab Instance__ always exports All levels of detail (LOD) available in the hierarchy for LOD groups. |
 | __Object(s) Position__    | __Convert to Linked Prefab Instance__ always resets the root object's transform during export. However, the Prefab maintains the global transform for the root object. |
 | __Animated Skinned Mesh__ | Check this option to export animation on objects with skinned meshes.<br/><br/>If unchecked, the FBX Exporter does not export animation on skinned meshes. |
-| __Compatible Naming__     | Check this option to control renaming the GameObject and Materials during export. <br/><br/>The FBX Exporter ensures compatible naming with Autodesk® Maya® and Autodesk® Maya LT™ to avoid unexpected name changes between Unity and Autodesk® Maya® and Autodesk® Maya LT™. During export the FBX Exporter replaces characters in Unity names as follows:<br/> - Replaces invalid characters with underscores ("_"). Invalid characters are all non-alphanumeric characters, except for colon (":").<br/> - Adds an underscore ("_") to names that begin with a number. - Replaces diacritics. For example, replaces "é" with “e”.<br/><br/>For FBX Model filenames, the FBX Exporter ensures that names do not contain invalid characters for the file system. The set of invalid characters may differ between file systems.<br/><br/>**Note:** If you have a Material with a space in its name, the space is replaced with an underscore ("_"). This results in a new Material being created when it is imported. For example, the Material named "Default Material" is exported as "Default_Material" and is created as a new Material when it is imported. If you want the exported Material to match an existing Material in the scene, you must manually rename the Material before exporting. |
+| __Compatible Naming__     | Check this option to control renaming the GameObject and Materials during export. <br/><br/>The FBX Exporter ensures compatible naming with Autodesk® Maya® and Autodesk® Maya LT™ to avoid unexpected name changes between Unity and Autodesk® Maya® and Autodesk® Maya LT™. During export the FBX Exporter replaces characters in Unity names as follows:<br/> - Replaces invalid characters with underscores ("\_"). Invalid characters are all non-alphanumeric characters, except for colon (":").<br/> - Adds an underscore ("\_") to names that begin with a number. - Replaces diacritics. For example, replaces "é" with “e”.<br/><br/>For FBX Model filenames, the FBX Exporter ensures that names do not contain invalid characters for the file system. The set of invalid characters may differ between file systems.<br/><br/>**Note:** If you have a Material with a space in its name, the space is replaced with an underscore ("_"). This results in a new Material being created when it is imported. For example, the Material named "Default Material" is exported as "Default_Material" and is created as a new Material when it is imported. If you want the exported Material to match an existing Material in the scene, you must manually rename the Material before exporting. |
 | __Don't ask me again__    | Check this option to use the same **Convert Option** properties and hide this window when converting to Linked Prefabs in the future. You can reset this option by turning on the **Show Convert UI** option under **Edit** > **Project Settings** > **Fbx Export** in Unity's top menu. |