Explicitly state that LDAP sync does not provision users

What does this MR do?

Our current LDAP synchronization docs mention this:

If you have configured LDAP to work with GitLab, GitLab can automatically synchronize users and groups. This process updates user and group information.

The definition of these words may lead some readers to assume that GitLab's LDAP synchronization also creates new GitLab users via LDAP which is not the case. The LDAP synchronization tasks (user and group sync) only affect existing GitLab users and groups.

Let's make this distinction explicit in the docs.

Related issues

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