[herald] Default msg should be in HTML, not markdown
Summary:
T2972
Depends on D13622
Default tg msg should no be in markdown. Edit comments to remove markdown references and better describe function.
Test Plan: npm test
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D13623