Add a little to ViewportTexture docs

(cherry picked from commit b820056b38)

doc/classes/ViewportTexture.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="UTF-8" ?>
 <class name="ViewportTexture" inherits="Texture" category="Core" version="3.0.4">
 	<brief_description>
+		Texture which displays the content of a [Viewport].
 	</brief_description>
 	<description>
 	</description>
@@ -12,6 +13,7 @@
 	</methods>
 	<members>
 		<member name="viewport_path" type="NodePath" setter="set_viewport_path_in_scene" getter="get_viewport_path_in_scene">
+			The path to the [Viewport] node to display. This is relative to the scene root, not to the node which uses the texture.
 		</member>
 	</members>
 	<constants>