Documentation Update: General LDAP Setup

The General LDAP Setup documentation has a line that leads to a lot of confusion for users:

Users added through LDAP take a licensed seat.

While this might be true for the lower tiers, it is not accurate for Ultimate and above who have Guest users.

In GitLab, a 'guest' is any users who isn't added to a group or project at a higher level than 'guest'. So just by signing in, with Ultimate, an LDAP user shouldn't take a seat.

We should update this documentation to clarify, so Ultimate users understand they can use Guest Users with LDAP.

See this Slack thread for more details.