Fix improper Doxygen inline comments

Summary:
The proper syntax for same line doxygen-comments is "//!<", but we prefer comments to be on the previous line.
http://www.doxygen.nl/manual/docblocks.html#memberdoc

This is a backport of Core PR17907

Test Plan: ninja doc-doxygen && firefox doc/doxygen/html/structutil_1_1_settings_span.html

Reviewers: #bitcoin_abc, majcosta, deadalnix

Reviewed By: #bitcoin_abc, majcosta, deadalnix

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8425