fix comments !< to !

Summary:
The style //!< is for documentation put *after* the member
being documented.
http://www.doxygen.nl/manual/docblocks.html#memberdoc

You can see how that's messing up the documentation of private
members here for example:
https://bitcoindoxygen.art/ABC/class_c_tx_mem_pool_entry.html

(Note there are still several files using //!< but correctly.)

Test Plan: ninja check all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5180