Clarify requirements for input monitoring permissions

[sarahmonster]

From https://github.com/octopusthink/mic-drop/issues/94:

A user pointed out that we only need Input Monitoring permissions for certain keyboard shortcuts.

My understanding is double-tap requires Input Monitoring, and possibly function keys. For other shortcuts, it's optional.

We should test and verify which shortcuts need it, and explain as much on the website and in the onboarding.