Add some man page cross-references

The old/deprecated servername callback should refer back to the
new/preferred early callback mechanism, as well as indicate that
it is superseded by the early callback.

The early callback should also mention the API for turning the
raw cipherlist octets from the client into usable data structures.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3338)