Please add documentation to ViewportContainer

**Your Godot version:**
3.2.2 stable

**Issue description:**
[ViewportContainer](https://docs.godotengine.org/en/stable/classes/class_viewportcontainer.html) is not documented right now, and using it cause lots of confusion.

What info I missing:

1. how ViewportContainer work when there more than one ViewportContainer on Scene
2. how Input bind to child Viewport scene
3. why changing manually Viewport "update" options has no effect (and other hidden features of ViewportContainer)
4. how to resize viewport correctly and info about shadow-atlas that ViewportContainer do not affect those options

Info like this should be added:

1. only "last" ViewportContainer (highter node_index `get_index()`) will work correctly when there more than one ViewportContainer on scene.
2. Input bind only to top ViewportContainer. Even when top ViewportContainer is hidden Input still bing to this ViewportContainer, to bing input to other ViewportContainer you have to move needed ViewportContainer on top `move_child()`.
Also any "top scene"(where ViewportContainer located) buttons or other input elements will lost all input events when ViewportContainer on scene.
3. on showing/hiding ViewportContainer node it set Viewport update option, after showing/hiding event changing Viewport update options will work.
4. to resize Viewport change its size after window resize, Shadow-Atlas have to be set to make shadows work

**URL to the documentation page:**
https://docs.godotengine.org/en/stable/classes/class_viewportcontainer.html