gio: Update bad cert error in accept-certificate and GTlsError docs
Looking at the docs for accept-certificate, it first mentions
G_TLS_ERROR_BAD_CERTIFICATE being returned when a TLS connection is established if no listener accepts the certificate, then mentions
G_TLS_ERROR_HANDSHAKE as being returned. Looking at both glib and glib-networking source, it seems that the former is correct, and the latter is never actually used.
This makes the documented error returned when the cert is rejected consistent with both self and reality.