Fix checksum file names and verification instructions

Explain how to find and verify the checksum algorithms provided with
each release. Also mention the use of shasum tools and give a small
usage example.

I slighly modified the original submission to use FreeBSD 13.1 in the
example. In other instances, simplify wording a bit to not repeat words
too often within a sentence.

PR: 266692
Event: Aberdeen Hackathon 2022