net_processing: Move comments to declarations

Summary:
This is a backport of core#21525
https://github.com/bitcoin/bitcoin/pull/21525/commits/1dd8ed7a8491e51b76eeb236b15b794d9254f674

Test Plan: NA. Only comments moved and blank lines deleted

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11506