(doc) Document key purging in the description of the ssh_authorized_key ...

...type

This commit puts an explanation of key purging over where people will likely be
looking for it. It also tightens up the description of the user type's
purge_ssh_keys attribute, and adds an example key to the ssh_authorized_key type
(since the way that type breaks up key lines is somewhat unintuitive).

The example public key is a dud, to prevent accidental access grants.

When you merge this commit, please resolve DOC-745.