Update EditableMesh.yaml and AssetService.yaml (#1018)

## Changes

Changed one reference of "content" in EditableMesh.yaml to link to [the
Content
datatype](https://create.roblox.com/docs/reference/engine/datatypes/Content).
Also added summaries for content parameters in AssetService.yaml to
better explain what they are.

Credit to
[@MrChickenRocket](https://devforum.roblox.com/u/MrChickenRocket) for
making [the bug report
here](https://devforum.roblox.com/t/feedback-on-editablemesh/3450653)
asking for clarification on Content.

## Checks

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- [x] This contribution was created in whole or in part by me, and I
have the right to submit it under the terms of this repository's open
source licenses.
- [x] I understand and agree that this contribution and a record of it
are public, maintained indefinitely, and may be redistributed under the
terms of this repository's open source licenses.
- [x] To the best of my knowledge, all proposed changes are accurate.

---------

Author: sammygrey

content/en-us/reference/engine/classes/AssetService.yaml

@@ -209,7 +209,9 @@ methods:
       - name: content
         type: Content
         default:
-        summary: ''
+        summary: |
+          Reference to asset content stored externally or as an object within the place, wrapping a single value
+          of one of the supported `Enum.ContentSourceType` values.
       - name: editableImageOptions
         type: Dictionary?
         default:
@@ -250,7 +252,9 @@ methods:
       - name: content
         type: Content
         default:
-        summary: ''
+        summary: |
+          Reference to asset content stored externally or as an object within the place, wrapping a single value
+          of one of the supported `Enum.ContentSourceType` values.
       - name: editableMeshOptions
         type: Dictionary?
         default:
@@ -319,7 +323,9 @@ methods:
       - name: meshContent
         type: Content
         default:
-        summary: ''
+        summary: |
+          Reference to asset content stored externally or as an object within the place, wrapping a single value
+          of one of the supported `Enum.ContentSourceType` values.
       - name: options
         type: Dictionary
         default: nil

content/en-us/reference/engine/classes/EditableMesh.yaml

@@ -21,7 +21,7 @@ description: |
   An `Class.EditableMesh` is displayed when it's linked to a new
   `Class.MeshPart`, through `Class.AssetService:CreateMeshPartAsync()`. You can
   create more `Class.MeshPart` instances that reference the same
-  `Class.EditableMesh` content, or link to an existing `Class.MeshPart` through
+  `Class.EditableMesh` `Datatype.Content`, or link to an existing `Class.MeshPart` through
   `Class.MeshPart:ApplyMesh()`.
 
   To recalculate collision and fluid geometry after editing, you can again call