doc: Add non-thread-safe note to FeeFilterRounder::round()
Summary:
Co-authored-by: MarcoFalke <falke.marco@gmail.com>
This is a backport of core#19268
Test Plan: Proof-reading, check that our implementations of FeeFilterRounder::round are similar enough that the comment applies.
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D9995