Update documentation

Add details of the use of PSS for signature algorithms.

Document SSL_get_peer_signature_nid() and SSL_get_peer_signature_type_nid().

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2301)