doc: Various validation doc fixups

Summary:

• Inline comments about faking chain data to avoid duplicating them

Other changes are either already fixed in Bitcoin ABC (typos) or not applicable (Segwit & RewindBlockIndex).

This is a backport of core#22121

Test Plan: proofreading

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12301