manuals: Fix some "missing section argument"

These were reported by mandoc -T lint ... as warnings.

Signed-off-by: Graham Percival <gperciva@tarsnap.com>
Reviewed by: mhorne, Alexander Ziaee <concussious.bugzilla@runbox.com>
Sponsored by: Tarsnap Backup Inc.
Pull Request: https://github.com/freebsd/freebsd-src/pull/1467