Update RichText onSetup docblock

[afercia]

The RichText onSetup docblock should be updated after recent changes:

gutenberg/packages/editor/src/components/rich-text/index.js

Lines 136 to 146 in 652a662

	/**
	* Handles the onSetup event for the TinyMCE component.
	*
	* Will setup event handlers for the TinyMCE instance.
	* An `onSetup` function in the props will be called if it is present.
	*
	* @param {tinymce} editor The editor instance as passed by TinyMCE.
	*/
	onSetup( editor ) {
	this.editor = editor;
	}

• it won't use an onSetup function passed via props any longer
• not sure it's setting up any event handler any longer
• it's only setting this.editor to the editor instance

[ellatrix]

Yes, this should be updated. This method will also go away once we update list operations.