MINOR: doc: update the manpage and usage message about -S

Add -S in the manpage, and update the usage message.

Should be backported to 1.9.
diff --git a/doc/haproxy.1 b/doc/haproxy.1
index f874986..7554f9d 100644
--- a/doc/haproxy.1
+++ b/doc/haproxy.1
@@ -173,6 +173,12 @@
 from the old process. Those sockets will then be used if possible instead of
 binding new ones.
 
+.TP
+\fB\-S <bind>[,<bind options>...]\fP
+In master-worker mode, create a master CLI. This CLI will enable access to the
+CLI of every worker. Useful for debugging, it's a convenient way of accessing a
+leaving process.
+
 .SH LOGGING
 Since HAProxy can run inside a chroot, it cannot reliably access /dev/log.
 For this reason, it uses the UDP protocol to send its logs to the server,