[docs] Improve commenting in ProcessGetData()
Summary:
This concludes backport of Core PR18808 [4/4]
https://github.com/bitcoin/bitcoin/pull/18808/commits/9847e205bf7edcac4c30ce4b6d62f482aa7bc1b7
Depends on D9085
Test Plan: ninja
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D9086