In DrRacket, when the cursor is on an identifier, you can hover over the big arrow at the top right corner of the definition window. A popup will appear showing

In DrRacket, when the cursor is on an identifier, you can hover over the big arrow at the top right corner of the definition window. A popup will appear showing basic usage of functions / macros without detailed explanation. However, you can click "read more..." which will open up the full documentation in a browser. Also, this documentation is installed locally and does not require the internet.