Previously, the documentation indicated that it was possible to call
g_tls_connection_handshake() after an initial handshake to trigger a
rehandshake, but only if TLS 1.2 or older is in use. However, there is
no documented way to ensure TLS 1.2 gets used. Nowadays, TLS 1.3 is used
by default.

I'm removing support for rehandshaking from glib-networking, as part of
a large refactoring where keeping rehandshakes would have entailed
significant additional complexity. So let's update the documentation to
indicate this is no longer ever supported. Applications should not
notice any difference.

Also, sync some previous handshake and rehandshake changes from
GTlsConnection to GDtlsConnection that were missed by mistake. I
try to remember to always update GDtlsConnection when touching
GTlsConnection documentation, but it's easy to forget.