Fix a slew of mdoc warnings/errors.
MFC requested by meeena in IRC.
share/man/man9/VOP_SETLABEL.9 and share/man/man9/hz.9 omitted since
they didn't exist in 13-STABLE.
share/man/man9/mod_cc.9 omitted as some of the changes are in text
not applicable to 13-STABLE.
(cherry picked from commit 320686674b3c8ce3ffe135c421a22d063917ee46)
Approved by: carlavilla (mentor)