man/system-or-user-ns.xml: explicitly refer to `PrivateUsers=` option
    
    It is not clear what "unprivileged user namespaces are available" means.
    It could mean either that they are only usable, that is, enabled in the kernel,
    or they have been enabled for the specific service. Referring to the
    `PrivateUsers=` options makes it clear that the latter is meant.
    
    (cherry picked from commit 34aee208b5d745fd6c90c09e3e6e91f2c2949514)