Provide some help for 'Input key event' screen

[jambl3r]

Developer TODO (don't remove)

• update documentation

This is probably part of a wider issue that the actions help page is very minimal and self-referential.

But regardless, there is no help button on the 'Input key event' screen, which makes things like understanding the difference between 'From device: No device' and 'From device: Virtual' pretty hard.

The parent issue had some confusion about this feature but once the user figured it out it massively improved the utility of the app for their use.