man: encourage user to read on about --tls-crypt

man: encourage user to read on about --tls-crypt

As suggested by krzee in trac #790, refer to the --tls-crypt option
form the --tls-auth section of the man page, to encourage users to
check out the --tls-crypt feature.

Trac: #790
Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <1482703334-18949-1-git-send-email-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13713.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 403dfe1bfdbdf6e5f8abac3401a96852562aec54)