docs/ctty.htm: add a useful URL to
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/docs/ctty.htm b/docs/ctty.htm
index 8f466cd..e7d2877 100644
--- a/docs/ctty.htm
+++ b/docs/ctty.htm
@@ -9,6 +9,8 @@
<p>Before looking at the Linux implementation, first a general Unix
description of threads, processes, process groups and sessions.
+</p><p>
+(See also <a href="http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap11.html">General Terminal Interface</a>)
</p><p>A session contains a number of process groups, and a process group
contains a number of processes, and a process contains a number
of threads.