Make it clear that this enum just defines ASCII control characters, and
there is no 'free space'. Also add comment showing which key combination
produces each control character.
Change-Id: I01884132842281cce6c13dd9398b1aeddadae58a
Reviewed-on: https://review.haiku-os.org/c/haiku/+/8899
Reviewed-by: nephele nephele <nep-git@packageloss.eu>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>