Add missing pieces of r315280

I moved this branch from github to a private server, and pulled from the
wrong one when committing r315280, so I failed to include two recent commits.
Thankfully, they were only cosmetic and were included in the review.
Specifically:

Add documentation, polish comments, and improve style(9).

Tested by: pho (r315280)
MFC after: 2 weeks
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D9791