doxygen: Fix member comments

Summary:
Trailing comments must be indicted with the caret //!<.

Backport of Bitcoin Core PR14331
https://github.com/bitcoin/bitcoin/pull/14331

Test Plan:

doxygen doc/Doxyfile

Reviewers: Fabien, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4173