Remove Heartbeat API Experimental Notices

[kirasong]

We took off the Experimental label in 3.9 [27981], but due to some remaining PHP docs, it's showing up here:
​http://developer.wordpress.org/reference/functions/wp_ajax_heartbeat/

Attached is a patch that gets rid of the experimental label everywhere else other than the long-polling in JS.

Related: #25272

[kirasong]

So, that patch​ was named as if it were going to be attached to the other ticket. Oops. But it's the correct one.

[wonderboymusic]

In 28872:

Remove remaining Heartbeat API experimental notices

Props DH-Shredder.
Fixes #28626.