docs: Move FindFilesToPrune{,Manual} doxygen comment
Summary:
[META] This is a pure comment commit.
They belong in the member declarations in the header file.
This is a backport of core#19927 [5/6]
https://github.com/bitcoin/bitcoin/pull/19927/commits/3756853b15902d63f4b5a3129e8b5d82e84e125b
Depends on D9823
Test Plan: NA
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D9824