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.

gio/gtlsconnection.c

@@ -757,8 +757,10 @@ g_tls_connection_get_rehandshake_mode (GTlsConnection       *conn)
  * the beginning of the communication, you do not need to call this
  * function explicitly unless you want clearer error reporting.
  * However, you may call g_tls_connection_handshake() later on to
- * rehandshake, if TLS 1.2 or older is in use. With TLS 1.3, this will
+ * rehandshake, if TLS 1.2 or older is in use. With TLS 1.3, the
- * instead perform a rekey.
+ * behavior is undefined but guaranteed to be reasonable and
+ * nondestructive, so most older code should be expected to continue to
+ * work without changes.
  *
  * #GTlsConnection::accept_certificate may be emitted during the
  * handshake.