[docs] add release notes for new unbroadcast transaction status

Summary:
This is a backport of Core PR18807 [1/9]
https://github.com/bitcoin/bitcoin/pull/18807/commits/dab298d9ab5a5a41685f437db9081fa7b395fa73

Note: the 3rd commit of this PR will not be backported, as it added a duplicate line later removed in PR19178

Test Plan: proof-reading

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9028