[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