Further specifics for global shortcuts implementation

[checkraisefold]

I think the following should be covered by the global shortcuts spec/documentation:

• What should be done when a shortcut is attempted to be registered that has a preferred trigger conflicting with an existing shortcut? Should it not be registered (not returned in response signal), registered with an empty trigger_description, or something like "unset" in the trigger_description?
• Should shortcuts be saved for relaunches based on app ID, or something else?

The above are relevant questions currently faced by both KDE and Hyprland's implementation of the spec.

[checkraisefold]

These are both really implied or implementation-specific intentionally I think