lock.9: describe lockmgr_{disowned,lock_flags,slock,unlock,xlock}

Describe and link the following functions in the lockgmr API:

• lockmgr_disowned
• lockmgr_lock_flags
• lockmgr_slock
• lockmgr_unlock
• lockmgr_xlock

This is not a complete update of lock.9 but at least covers all the main
lock operations.

Reviewed by: gbe, kib
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D45689