Rebrand comments from C++ source files

Summary:
As per title.

Ref T1672.

Test Plan:
Read the comments.

arc lint

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Maniphest Tasks: T1672

Differential Revision: https://reviews.bitcoinabc.org/D9804