Clarify --block-ipv6 intent and direction.

Clarify --block-ipv6 intent and direction.

--block-ipv6 is a fairly special-purpose option, and only blocks packet
in the client->server direction.  This is implied by not ever mentioning
the other direction in the existing documentation, but not written down.

Make this explicit, avoid confusion.

Also, point why this option exist (avoid IPv6 leakage from dual-stacked
clients around IPv4-only VPN offerings).

Trac: #1351

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Richard Bonhomme <tincanteksup@gmail.com>
Message-Id: <20201225164214.22771-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21407.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 8a8ee283aa7a4b409a9dafc082a6c65b5539308b)