Add SSL_get_extms_support documentation.

Document SSL_get_extms_support().

Modify behaviour of SSL_get_extms_support() so it returns -1 if the
master secret support of the peer is not known (e.g. handshake in progress).
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>