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

Add description of new libmach.
diff --git a/man/man3/getpid.3 b/man/man3/getpid.3
index c2075df..a1cc50f 100644
--- a/man/man3/getpid.3
+++ b/man/man3/getpid.3
@@ -31,7 +31,7 @@
 .SH SOURCE
 .B /sys/src/libc/9sys
 .SH SEE ALSO
-.IR intro (2),
+.IR intro (3),
 .IR cons (3),
 .IR proc (3)
 .SH DIAGNOSTICS