Add note for some usages of Plane in APIs

Some old APIs that may expect a Vector4 instead require a Plane: this
is pretty confusing especially with tangents as one could interpret
the API as wanting a value directly representing the tangent plane when
in reality the Plane type is just used to carry one Vector3 and one
extra sign component.

This commit attempts to clear any potential confusion by adding a note
to the APIs using Planes in such a way.

doc/classes/MeshDataTool.xml

@@ -318,6 +318,7 @@
 			<param index="1" name="tangent" type="Plane" />
 			<description>
 				Sets the tangent of the given vertex.
+				[b]Note:[/b] Even though [param tangent] is a [Plane], it does not directly represent the tangent plane. Its [member Plane.x], [member Plane.y], and [member Plane.z] represent the tangent vector and [member Plane.d] should be either [code]-1[/code] or [code]1[/code]. See also [constant Mesh.ARRAY_TANGENT].
 			</description>
 		</method>
 		<method name="set_vertex_uv">