Tighten up SSL_get1_supported_ciphers() docs

This function is really emulating what would happen in client mode,
and does not necessarily reflect what is usable for a server SSL.
Make this a bit more explicit, and do some wordsmithing while here.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4284)