Update documentation of g_tls_connection_handshake() again

I made a mistake when last updating the documentation in 94a99ae9. I wrote that, with TLS 1.3, this would perform a rekey instead of a rehandshake. In fact, that's only true for client connections. For server connections, it's a no-op.

I was a bit nervous about how to document the behavior anyway, because we really don't know what behavior will be reasonable with non-GnuTLS crypto backends. This behavior is reasonable for the GnuTLS backend, but might not necessarily make sense for OpenSSL. Ideally, we would discourage API users from doing things which could have unexpected effects, so instead of documenting what the GnuTLS backend does, I think it'd be better to document that this is "undefined but not dangerous," since of course we want to make sure that existing code that doesn't know about TLS 1.3 is not broken.