PopupMenu could use clarification on shortcuts and accelerators

1. There's very little in the documentation to clarify the difference between `add_shortcut()`, `set_item_shortcut()`, and `set_item_accelerator()`. ShortCuts seem to be able to bind more actions to them than accelerators, but using `InputEventAction` as `ShortCut.shortcut` will create an extremely verbose description by default.

2. I couldn't find anywhere in the documentation regarding the limitations on "global" scope of shortcuts/accelerators. It seems that PopupMenu might not even be able to do this by itself without living inside a helper control such as `MenuButton`. Making a PopupMenu the child of a standard Button will not activate the shortcut, no matter which control seems to be in focus. This may be by design. If it is, it should be documented appropriately in `PopupMenu` and anywhere else global ShortCuts/accelerators are used. If it's not intended functionality, I'll file an issue in [godotengine/godot](https://github.com/godotengine/godot).

Related to #2760