mutex.9: Misc updates and minor improvements.

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