[herald] Improve generateMocks script calling

Summary:
T2972

Depends on D13627

Add commenting to generateMocks.js explaining how to run the script. Change name of script to match name of file.

Test Plan:
npm run generateMocks
npm run generateMocks true

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13639