Add longer description to ViewportTexture doc

doc/classes/ViewportTexture.xml

@@ -4,6 +4,8 @@
 		Texture which displays the content of a [Viewport].
 	</brief_description>
 	<description>
+		Displays the content of a [Viewport] node as a dynamic [Texture]. This can be used to mix controls, 2D, and 3D elements in the same scene.
+		To create a ViewportTexture in code, use the [method Viewport.get_texture] method on the target viewport.
 	</description>
 	<tutorials>
 	</tutorials>