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

Add description of new libmach.
diff --git a/man/man3/bin.3 b/man/man3/bin.3
index 12ca065..819f29e 100644
--- a/man/man3/bin.3
+++ b/man/man3/bin.3
@@ -82,7 +82,7 @@
 and
 .I bingrow
 allocate large chunks of memory using
-.IR malloc (2)
+.IR malloc (3)
 and return pieces of these chunks.
 The chunks are
 .IR free 'd
@@ -91,7 +91,7 @@
 .SH SOURCE
 .B /sys/src/libbin
 .SH SEE ALSO
-.IR malloc (2)
+.IR malloc (3)
 .SH DIAGNOSTICS
 .I binalloc
 and