man: document that separate /usr/local/ must not be used for config

man: document that separate /usr/local/ must not be used for config

Since we document /usr/local/lib/systemd/ and other paths for various things,
add notes that this is not supported if /usr/local is a separate partition. In
systemd.unit, I tried to add the footnote in the table where
/usr/local/lib/systemd/ is listed, but that get's rendered as '[sup]a[/sup]'
with a mangled footnote at the bottom of the table :( .

Also, split paragraphs in one place where the subject changes without any
transition.

Follow-up for 02f35b1c905ac63ba62f94efebf858412e961fc1.
Replaces https://github.com/systemd/systemd/pull/33231.