Document that RSA_SIGN can also request TLS 1.2 signatures

Document that RSA_SIGN can also request TLS 1.2 signatures

Ever since we support TLS 1.2 (OpenVPN 2.3.3+), the RSA_SIGN might not
only request MD5-SHA1 'TLS signatures', but also other variants.
Document this by updating the implementation hints, and explicitly
stating that we expect a PKCS#1 1.5 signature.

Trac: #764
Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1482705505-20302-1-git-send-email-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13714.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 1e36b814073c0f56c77e4922cc105f00b8558e7e)