Add scant documentation for msleep.

share/man/man9/Makefile

@@ -73,7 +73,7 @@ MLINKS+=posix4.9 p1003_1b.9
 MLINKS+=psignal.9 gsignal.9 psignal.9 pgsignal.9
 MLINKS+=rtalloc.9 rtalloc1.9 rtalloc.9 rtalloc_ign.9
 MLINKS+=sleep.9 tsleep.9 sleep.9 wakeup.9 sleep.9 wakeup_one.9
-MLINKS+=sleep.9 asleep.9 sleep.9 await.9
+MLINKS+=sleep.9 asleep.9 sleep.9 await.9 sleep.9 msleep.9
 MLINKS+=buf.9 bp.9
 MLINKS+=spl.9 spl0.9
 MLINKS+=spl.9 splbio.9 spl.9 splclock.9 spl.9 splhigh.9 spl.9 splimp.9

share/man/man9/sleep.9

@@ -30,6 +30,7 @@
 .Dt SLEEP 9
 .Sh NAME
 .Nm sleep ,
+.Nm msleep ,
 .Nm tsleep ,
 .Nm asleep ,
 .Nm await ,
@@ -42,6 +43,8 @@
 .Ft int
 .Fn tsleep "void *ident" "int priority" "const char *wmesg" "int timo"
 .Ft int
+.Fn msleep "void *ident" "mtx_t *mtx" "int priority" "const char *wmesg" "int timo"
+.Ft int
 .Fn asleep "void *ident" "int priority" "const char *wmesg" "int timo"
 .Ft int
 .Fn await "int priority" "int timo"
@@ -108,6 +111,22 @@ possible, and
 is returned if the system call should be interrupted by the signal
 .Pq return Dv EINTR .
 .Pp
+.Nm Msleep
+is a variation on tsleep.  The parameter
+.Ar mtx
+is a mutex, which will be exited before sleeping, and entered before
+.Nm msleep
+returns.  If
+.Ar pri
+includes the
+.Dv PDROP
+flag, the
+.Ar mtx
+parameter will not be entered before returning.  The mutex is
+used to ensure that a condition can be checked atomicly, and
+that the current process can be suspended without missing a
+change to the condition, or an associated wakeup.
+.Pp
 .Nm Asleep
 implements the new asynchronous sleep function.  It takes the same arguments
 as