aio / timers: De-document -clock

Commit 6d327171 "aio / timers: Remove alarm timers" has issues:

1. It silently ignores -clock for backward compatibility.
Incompatible change: -clock help no longer terminates the program.
Tolerable.

2. Failed to update option documentation.  In particular, -help still
advises users to try -clock help for available timers.  Drop all
documentation on -clock.

3. The 'query-alarm-clock' example in docs/writing-commands.txt no
longer works, and needs to be redone.  Can't do that right now, so I
just stick in a FIXME.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>