Quick Documentation Lookup allows you pop-up a little window that shows you a documentation of method.
First of all you have to download Java API from here.
Then press "Project Structure" button, click label SDKs, "Documentation Paths" tab and add path by choosing path where you saved Java API.
Put your cursor on method and invoke the pop-up window by pressing CTRL+Q.
If you wanna find more information just click an up arrow in the pop-up window, it will open a browser and take you directly to the method.
So, that's a quik documentaion lookup.
Enjoy. :)