man: document system-systemd\x2dcryptsetup.slice
    
    As discussed in
    https://github.com/systemd/systemd/pull/14235/commits/1dc85eff1d0dff18aaeaae530c91bf53f34b726e#r606821495,
    follow-up for commit 1dc85eff1d0dff18aaeaae530c91bf53f34b726e.
    
    (cherry picked from commit 5c91fdf3f832a45db35b207da650c4aa12fafa75)