man: fix description of %N in systemd.unit(5)

man: fix description of %N in systemd.unit(5)

The description in the man page disagreed with the code. Let the code win,
since if anybody is using this, they are more likely to depend on actual
behaviour rather than the docs. (In Fedora workstation installation there's
only one use, and it doesn't make much sense either way: SyslogIdentifier=%N
in xfs_scrub@.service.)

Also adds dots at the end everywhere, because we have multiple sentences in
some explanations, so we need dots.