clarify the return value of monmwait.

Reference: /n/atom/patch/applied/monmwaitrv
Date: Sun Jun  8 11:24:31 CES 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/man/9/monmwait	Sun Jun  8 11:24:17 2014
+++ /sys/man/9/monmwait	Sun Jun  8 11:24:17 2014
@@ -20,7 +20,9 @@
 to change from
 .B oldval
 in the most power- and bus-cycle efficient way afforded by
-the architecture.
+the architecture and return possibly unchanged value pointed to
+by
+.IR adr .
 .PP
 Since no lock is involved,
 .I monmwait
@@ -35,8 +37,10 @@
 means in the
 same cacheline.
 .PP
-The minimal correct implementation would be a no-op.
-On x86 machines, the implementation uses
+The minimal correct implementation would be returning
+the value pointed to by
+.IR adr .
+On recent x86 machines, the implementation uses
 the
 .SM MONITOR
 and
@@ -48,8 +52,8 @@
 while waiting for release to keep the fans from spooling up:
 .IP
 .EX
-while(!active.thunderbirdsarego)
-	monmwait(&active.thunderbirdsarego, 0);
+while(monmwait(&active.thunderbirdsarego, 0) == 0)
+	;
 .EE
 .PP
 .SH SOURCE