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