docs: stop mentioning insecure / broken SASL mechanisms

docs: stop mentioning insecure / broken SASL mechanisms

We don't need to go to the trouble of telling users about existance of
insecure SASL mechanisms only to then say that they shouldn't be used.
We should only tell people about the GSSAPI mechanism for TCP sockets.

For the SCRAM mechanism we should be telling people about the SHA256
variant only, and also warning that the password database stores the
passwords in clear text.

Reviewed-by: Erik Skultety <eskultet@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>