Provide better documentation for SSL_get_servername()

Provide better documentation for SSL_get_servername()

The behaviour of SSL_get_servername() is quite complicated and depends on
numerous factors such as whether it is called on the client or the server,
whether it is called before or after the handshake, what protocol version
was negotiated, and whether a resumption was attempted or was successful.

We attempt to document the behavior more clearly.

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/10018)