net, net_processing: move NetEventsInterface method docs to net.h

Summary:
Move only, comment only.

Partial backport of core#20811:
https://github.com/bitcoin/bitcoin/pull/20811/commits/0d246a59b606c51728d10cb70004a6eedb951bca

Ref T1696.

Depends on D10881.

Test Plan: Read the comments

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D10882