Rewrite page(2) references to page(3).

Add description of new libmach.

man/man3/sleep.3

@@ -27,7 +27,7 @@ Sleep returns \-1 if interrupted, 0 otherwise.
 causes an
 .B alarm
 note (see
-.IR notify (2))
+.IR notify (3))
 to be sent to the invoking process after the number of milliseconds
 given by the argument.
 Successive calls to
@@ -39,7 +39,7 @@ the alarm clock.
 .SH SOURCE
 .B /sys/src/libc/9syscall
 .SH SEE ALSO
-.IR intro (2)
+.IR intro (3)
 .SH DIAGNOSTICS
 These functions set
 .IR errstr .