[doc] add notes on release notes

Summary:
The procedure of writing notes to diff-specific files to avoid conflicts does not seem useful for Bitcoin ABC, as we don't take as long for reviews as Core does.

This is a backport of Core PR15243

Test Plan: Proof-reading.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8434