[e.cash] Update the Heartbeat instructions for the next upgrade

Summary: We are adding a new window to the RTT filter, so we need instructions on how to properly handle the transition and the new formulae.

Test Plan:

npm run dev

Check the content is correct.

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D18773