Add physics limitations to 3.x pause_mode

[chucklepie]

Your Godot version:
3.2.3

Issue description:
Refer to comments in godotengine/godot#47326

When pause_mode is set to true, _process() and _physics_process() are still called for nodes with pause_mode set to 'process', i.e. they can still run and bodies can still move. However from what I believe they stop colliding and no collision signals are called. This is because the physics world can only be processed or paused as a whole.

It would be good if the pausing games section linked below contained some kind of section detailing this, i.e. the limitations of pause mode on collision handling and how Physics2DServer.set_active(true) can be used as a partial solution. As it stands, this not being mentioned gives a false pretence that a node in pause mode can function normally.

URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/tutorials/misc/pausing_games.html

and also,
https://docs.godotengine.org/en/stable/classes/class_scenetree.html#class-scenetree-property-paused