Add release notes about salvage changes

Summary:
Completes backport (11/11) of core PR18918:
https://github.com/bitcoin/bitcoin/pull/18918/commits/84ae0578b6c68dda145ca65fef510ce0fdac0d7b

Depends on D8597.

Test Plan: Read it.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8598