doc: add assumeutxo notes

Summary:
This is a backport of core#23154
with updates from core#25667 and core#25740

Depends on D14655

Test Plan: proof-reading

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14656