Docs: Improve JS Docs for `wp-includes/js/heartbeat.js`.

Most of the documentation was already present. This change achieves the following:

* Fixes the formatting for most doc blocks.
* Adds `@since` tags to all functions.
* Makes sure the heartbeat public methods are correctly added to the `wp.heartbeat` namespace.

Props andizer, jjcomack, carolinegeven.
Fixes #42675.

Built from https://develop.svn.wordpress.org/trunk@42389


git-svn-id: https://core.svn.wordpress.org/trunk@42218 1a063a9b-81f0-0310-95a4-ce76da25c4cd