document gssapi-keyex authentication method

[calestyo]

Hi.

Apparently, with the patchset there comes another authentication method "gssapi-keyex".

1. It should be documented what it actually does (in contrast to gssapi-with-mic).

2. How it's activated on client and server side, i.e. all the other auth methods have their own option that enables/disables it (e.g. PubkeyAuthentication) while gssapi-keyex seems to be enabled whenever GSSAPI based KEX is enabled.

3. It should be documented as a value for the PreferredAuthentications and AuthenticationMethods.

4. Possibly, it would also make sense to alter the default of PreferredAuthentications to include gssapi-keyex when GSSAPI KEX is eanbled?

Cheers,
Chris.

btw: Still not possible to report issues at https://github.com/gss-openssh/openssh-portable

[calestyo]

Forwarded to: gss-openssh/openssh-portable#3