[doc] Update unbroadcast description in RPC results
Summary:
This is a backport of Core PR18807 [8/9]
https://github.com/bitcoin/bitcoin/pull/18807/commits/8f30260a67166a6ab7c0f33f7ec1990d3c31761e
Test Plan: src/bitcoin-cli help getrawmempool
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D9035