sleep(9): recover comment with the right reference

sys/man/9/sleep

@@ -91,6 +91,14 @@ structure, for example:
 .IP
 .B tsleep(&up->sleep, return0, nil, 10);
 .PP
+Both
+.I sleep
+and
+.I tsleep
+can be interrupted by
+.IR postnote
+(see
+.IR kproc (9)).
 .SH SOURCE
 .B /sys/src/9/port/proc.c
 .br