doc: add explanations for assert in index and magic numbers in test

Summary:
These were suggested in review of #23365

Co-authored-by: John Newbery <john@johnnewbery.com>

This is a backport of core#23777

Depends on D11299

Test Plan: NA - comments only

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11303