mutex.9: Misc updates and minor improvements.
* mtx_spinlock_ex() and mtx_spinlock_sh() do not exist anymore. mtx_spinlock()
replaced both.
* Add MLINKS for mtx_lock(), mtx_spinlock_try() and mtx_spinunlock().
* Use .Fn for function name mtx_lock_ex_quick().
* Add some text describing mtx_lock(), mtx_spinlock_try() and mtx_spinunlock().
* Start a sentence with capital letter but avoid starting with a function name.
* Add a missing .Fa.
* Add a cross reference to sleep(9) in the SEE ALSO section.
* Bump date.
Reviewed-by: swildner