Change "Return" to "Returns" where necessary in XML documentation

In many of the XML files it had been noted that when the documentation
refers to a return value, both "Return" and "Returns" are used. This
has now been fixed to only say "Returns".

Fixes #28867

(cherry picked from commit 426bcc9941)

doc/classes/PoolVector3Array.xml

@@ -82,7 +82,7 @@
 			<return type="int">
 			</return>
 			<description>
-				Return the size of the array.
+				Returns the size of the array.
 			</description>
 		</method>
 	</methods>