cam: Add doxygen docs to cam_sim_alloc

Add description for what each of the parameters are to the cam_sim_alloc
call. Add some additional context for the mtx and queue parameters to
explain what special values passed in mean.

MFC After: 3 days
Reviewed by: mav@
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30115

(cherry picked from commit cb5880594387d5b07c5d580c4aa1b633947a6046)