doc: Add release notes for -avoidpartialspends

Summary:
This is a partial backport of Core PR12257 : https://github.com/bitcoin/bitcoin/pull/12257/commits/232f96f5c8a3920c09db92f4dbac2ad7d10ce8cf

Depends on D3394

Test Plan: Read the notes, maybe?

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3398