top: Polish key bindings in usage and manual

Organize key bindings by ascii(7) for consistency and maintainability,
mark them as Interactive Commands, wordsmith them, and sync their
organization between the manual and help screen.

MFC after: 3 days
PR: 282734
Fixes: c8aa5e526 (move command mapping to commands.c)
Reviewed by: imp, mhorne, Jim Brown <jpb@jimby.name>
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D49462

(cherry picked from commit 0a85254d5a33800600477ce57fbaab64591aa6ea)