If you are generating reference documentation for one or more Java Projects, you will probably want to make the generated documentation available in your Java Development Environment so that developers can get instant access to documentation when referencing the documented Java Projects.
In order for your documentation to be accessible to users of your Java Projects your documentation must be published in a reachable location. That location might be a folder on the local machine (e.g. "c:\program files\mycompany\myproduct\docs"), or an intranet or internet url (e.g. http://www.mycompany.com/docs/api/v1). You should publish all the files in the output directory specified in your Document! X Project.
When you add a reference to a Java Project you have created and published documentation for you should identify the url at which you have published the generated documentation so that the Java Development Environment can find the documentation. The way in which you do that will depend on the Java Development Environment, but it will typically be a "Javadoc location" property where you add the external library reference to documented Java Project.
The location you specify should be the root url, not a specific page name, e.g. http://www.mycompany.com/docs/api/v1
Whenever in code you reference one of the documented types or members, you will be able to access tooltip help about the type of member. The method to do this will depend on the Java Development Environment you are using, but usually you can do that just by hovering your mouse over the type / member reference in code and the tooltip help will appear.
Your Java Development Environment should also allow you to directly access the full documentation for the type or member under the cursor - either from the "Full Documentation" link in the tooltip help or by pressing Shift+F1.