Re-layout comments in ProcessGetBlockData and ProcessGetData

Summary:
This is a follow-up for a suggestion in D2606. Due the the functions
refactoring, blocks of comment ended up left-shifted and the layout can
be updated to look better.

Depends on D2607

Test Plan:
Read the comments

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2626