Add enc, comment and options for ssh_auth.absent

These are present for ssh_auth.present, but not for ssh_auth.absent.
This poses a problem further into the code in ssh.check_key (or rather
_refine_enc() in the ssh module) when it is validating the encryption
type.

Fixes #8043.
Argument documentation

And some copyedit.
More copyedit
Merge pull request #8046 from holmboe/issue-8043

Add enc, comment and options for ssh_auth.absent