Add documentation for Skeleton2D

Signal bone_setup_changed remains undocumented. I took a quick look at
the cpp code, but its purpose remained unclear to me. If anyone can
steer me in the right direction, I'm happy to flesh this out.

(cherry picked from commit 9151992432)

doc/classes/Skeleton2D.xml

@@ -4,6 +4,7 @@
 		Skeleton for 2D characters and animated objects.
 	</brief_description>
 	<description>
+		Skeleton2D parents a hierarchy of [Bone2D] objects. It is a requirement of [Bone2D]. Skeleton2D holds a reference to the rest pose of its children and acts as a single point of access to its bones. 
 	</description>
 	<tutorials>
 		<link>https://docs.godotengine.org/en/latest/tutorials/animation/2d_skeletons.html</link>
@@ -15,19 +16,21 @@
 			<argument index="0" name="idx" type="int">
 			</argument>
 			<description>
+				Returns a [Bone2D] from the node hierarchy parented by Skeleton2D. The object to return is identified by the parameter [code]idx[/code]. Bones are indexed by descending the node hierarchy from top to bottom, adding the children of each branch before moving to the next sibling.
 			</description>
 		</method>
 		<method name="get_bone_count" qualifiers="const">
 			<return type="int">
 			</return>
 			<description>
-				Returns the amount of bones in the skeleton.
+				Returns the number of [Bone2D] nodes in the node hierarchy parented by Skeleton2D.
 			</description>
 		</method>
 		<method name="get_skeleton" qualifiers="const">
 			<return type="RID">
 			</return>
 			<description>
+				Returns the [RID] of a Skeleton2D instance.
 			</description>
 		</method>
 	</methods>