misc package validation doc improvements

Summary:
This is a backport of core#21800 [1/9]
https://github.com/bitcoin/bitcoin/pull/21800/commits/f95bbf58aaf72aab8a9c5827b1f162f3b8ac38f4

Test Plan: proof-reading

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12154