camcontrol(8): Clean up references to removed symbols

Patch the manpage to remove references to devq_openings and devq_queued.
Document the allocated tag that has been added in the same commit. The
relevant code change was committed as r271588
(959ec2581b23aefebb3bae26c527c1b25834a9e3) by mav@.

PR: 223651
Reported by: Bertrand Petit <bsdpr@phoe.frmug.org>

(cherry picked from commit d4cf7dc0df289ad0e16b5b40b076311c71e663dc)