Studio 3 simplified greatly the concept of adding JavaScript content assist. Rather than a references view that needed to be constantly updated, we've instead allowed the user to drop content assist files into an existing project to get good code assist that way.
The library you are using must have either a JSCA 1.0 Specification file, be documented using the ScriptDoc spec, be documented using VSDoc for JavaScript, or have a Aptana Studio 2.X code assist XML file. If using the Studio 2.X XML code assist file, you need to change the file extension to .sdocml.
An error occurred while parsing the documentation XML file.
<api> cannot be a child of <#document>
Element stack trace
===================
<#document>
<api version="1.0"/>
</#document>
We've moved away from supporting code assist via a plug-in for a particular IDE (Aptana). Now we generate a type script definition file for the ArcGIS API for JavaScript. Several development enviroments like Jet Brains and Visual Studio support the TypeScript definitions. Details can be found in the Esri jsapi-resources Github repo: jsapi-resources/typescript at master · Esri/jsapi-resources · GitHub