versionbits: correct doxygen comments
Summary:
This is a partial backport of core#19438
https://github.com/bitcoin/bitcoin/pull/19438/commits/36a4ba0aaaa9b35185d7178994e36bc02cca9887
Test Plan: NA
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D12218