txorphanage: comment improvements

Summary:
This is a backport of core#21148 [14/14]
https://github.com/bitcoin/bitcoin/pull/21148/commits/5e50e2d1b95e7ca7709a9671ab21f1164b8d0cb8

Depends on D11492

Test Plan: NA (only comments were changed)

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11493