fcntl(2): Document interrupt/restart for file locks.

Since r302216, thread suspension causes advisory file locks to restart
(instead of continuing to wait) and for a long time SA_RESTART has
affected advisory file locks. These are both not compliant to POSIX.1.

To clarify that restarting means something, add a paragraph about fair
queuing. Note that the network lock manager does not implement fair
queuing.

Reviewed by: kib (previous version)
Approved by: re (gjb)