Update documentation for places to receive InputEventShortcut

[anniryynanen]

I didn't test it myself, but https://docs.godotengine.org/en/latest/tutorials/inputs/inputevent.html#how-does-it-work and https://docs.godotengine.org/en/latest/classes/class_node.html#class-node-private-method-shortcut-input say so.

[Mickeon]

Commit and title could say something different. They read as if this is a bug being reported, instead of a documentation fix.

[Sauermann]

The changes fix the previously outdated statement.

[akien-mga]

Thanks!