Update the message callback documentation
Update the message callback documentation

Update the message callback documentation to cover the new inner content
type capability. Also major update of the documentation which was very out
of date.

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3408)
openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl