Some cleanup for #2871
* Keyboard Settings dialog: improve error message wording.
* Keyboard Settings dialog: use log.debugWarning not log.error when logging that no NVDA key was selected.
* Keyboard Settings dialog: use not rather than literal false equality when checking if no checkboxes are checked. Just a bit shorter.
* user guide: re-word the paragraph about the error message when no NVDA key is selected.
