man: systemctl: clarify that --lines=0 is allowed (#10375)

man: systemctl: clarify that --lines=0 is allowed (#10375)

The term “positive” is often read to exclude 0 (though “strictly
positive” is sometimes used to clarify this), so let’s explicitly state
that --lines=0 is legal and completely disables journal output.

Motivated by an answer on StackExchange [1].

[1]: https://unix.stackexchange.com/a/475068/44049