The htmlarea stuff itself sits in acs-templating/www/resources/htmlarea.
There's a acs-templating parameter, UseHtmlAreaForRichtextP, which controls whether richtext widgets use the htmlArea.
This parameter defaults to 0 (off), because we don't yet have a feature that lets the user choose to turn off htmlArea, or switch between htmlArea and normal textarea input, and htmlArea doesn't work on all browsers.
For more documentation, see template::widget::richtext.
This is on HEAD, meaning 5.1.
For those interested in back-porting, there's also a change in acs-subsite/www/resources/core.js, and to www/blank-master.tcl/adp.