add troubleshooting section with known issues

+ start adding terminology section for prefabs.

Author: vkovec

com.unity.formats.fbx/Documentation~/TableOfContents.md

@@ -5,3 +5,4 @@
 * [Integrating Unity with 3D modeling software](integration)
 * [Setting FBX Export options](options)
 * [Developer's guide](devguide)
+* [Troubleshooting](troubleshooting)

com.unity.formats.fbx/Documentation~/prefabs-overview.md

@@ -15,4 +15,20 @@ Since Prefab Variants allow you to override its base Prefab's properties, you ca
 
 This type of connection is easy to maintain.
 
-* For information on Model Prefabs in general and how to create FBX Linked Prefabs from FBX files, see [Working with FBX Linked Prefabs](nested-prefabs.md). If you are upgrading from a previous version of Unity, you can also [convert any existing](nested-prefabs.md#conversion) Linked Prefabs you may have to the new FBX Linked Prefabs.
+* For information on Model Prefabs in general and how to create FBX Linked Prefabs from FBX files, see [Working with FBX Linked Prefabs](nested-prefabs.md). If you are upgrading from a previous version of Unity, you can also [convert any existing](nested-prefabs.md#conversion) Linked Prefabs you may have to the new FBX Linked Prefabs.
+
+
+## Terminology
+
+* Model Prefab: Generated on import of an FBX file. A Model Prefab is a read-only representation of the FBX contents, which updates when the FBX file is modified.
+
+* Prefab Variant: A Prefab generated from another Prefab (either Regular, Model, or from another Variant). It will inherit all data from the base Prefab, allowing additional modifications without affecting the base.
+                  For example, you can create a Variant of a button Prefab which has a different color but otherwise the same functionality.
+
+* Linked Prefab: A Regular Prefab containing an FbxPrefab component which connects to an FBX file. 
+                 The Prefab updates automatically when the FBX file is modified. Components are maintained and if object names or hierarchies change in the FBX file,
+                 it is possible to remap the objects by name on import.
+                 This is specific to previous versions of the FBX exporter, and requires Unity 2018.2 or lower.
+
+* FBX Linked Prefab: A Prefab Variant with a Model Prefab as its base. This feature replaces Linked Prefabs in Unity 2018.3 and above.
+                     A special component is no longer required, as FBX Linked Prefabs use only the Unity Prefab features.

com.unity.formats.fbx/Documentation~/troubleshooting.md

@@ -0,0 +1,38 @@
+# Troubleshooting
+
+## Known issues
+
+* When installing a new version of the FBX Exporter package after using version 1.3.0f1 or earlier, the link between Assets and FbxPrefab components may be lost. See [Updating from 1.3.0f1 or earlier](#Repairs_1_3_0f_1) for repairing instructions.
+
+* The FBX Exporter package does not support exporting `.asset` files.
+
+## Known issues
+
+* Bind pose of animated skinned mesh is lost on export. For example, if you export an animated skinned mesh from Unity and import it into Autodesk® Maya® and Autodesk® Maya LT™ you will not be able to set the character into the bind pose using the **Rigging** > **Skin** > **Go to Bind Pose** command.
+
+* Animated skinned meshes may not export with the correct skinning if they are not in the bind pose on export (that is, not being previewed in the Animation or Timeline windows, and the original Rig's FBX must not contain animation)
+
+* For skinned meshes all bones must be descendants of the root bone. For example, if the root bone is "hips" and the right leg for the same skinned mesh is not a descendant of hips, export will fail.
+
+## Known issues
+
+* Name or path changes are ignored when converting a Model instance
+
+## Known issues
+
+* Exporting animation only for animated lights and cameras is not currently supported
+
+
+----
+
+* Converting hierarchies with UI components will break UI
+    * FIX: prepare hierarchy so that it has no UI elements before converting then add it back afterwards
+    
+* In some situations, when overwriting an FBX file on export. If the FBX file is used by an FBX Linked Prefab instance, it may lead to unexpected results such as additional objects added to the hierarchy.
+    * FIX: recommended not to overwrite an FBX unless certain that it is not used by a Prefab Variant
+    
+* Converting a Tree primitive makes the Tree no longer editable.
+    * FIX: make sure to only convert when finished editing the tree. Otherwise click undo after converting to return to previous state where tree was editable.
+    
+* Trail and line particles lose material after being converted
+    * FIX: re-apply the lost materials after converting