Documentation improvements for assumeutxo

Summary:
This is a partial backport of core#27746
https://github.com/bitcoin/bitcoin/pull/27746/commits/0ce805b632dcb98944a931f758f76f530f5ce5f2

Depends on D17535

Test Plan: proof-read

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17536