Unify similar method descriptions

Notably:

* `Packed*Array.size()` and `Array.size()`.
* Shared methods of `Transform2D` and `Transform3D`.
* Shared methods of `Vector2`, `Vector3`, and `Vector2i`.

This reduces the Deja Vu when translating the class reference :)

doc/classes/PackedInt32Array.xml

@@ -127,7 +127,7 @@
 		<method name="size" qualifiers="const">
 			<return type="int" />
 			<description>
-				Returns the array size.
+				Returns the number of elements in the array.
 			</description>
 		</method>
 		<method name="slice" qualifiers="const">