Improve --tls-cipher and --show-tls man page description

Improve --tls-cipher and --show-tls man page description

As reported in trac tickets #304, #358 and #359 (and possibly more), the
usage and interpretation of --tls-cipher (and --show-tls) is tricky.  This
patch extends the man page to explain those a bit better and point out
that --tls-cipher is an expert feature (i.e. easy to get wrong).  Also add
a notice to the --show-tls output, referring to the man page explanation.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1430840857-6123-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9651
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 5f66f907cfc57b89110c08e50c7aab228e090911)

Conflicts:
	doc/openvpn.8
	src/openvpn/ssl_polarssl.c