Android Studio 2.x.x
Moved under Editor -> General
Older Versions
Using windows 7 and Android Studio 1.0.x it took me a while to figure out the steps provided in the answer.
To help further visitors save some time, here is the way I did it:
Go to File -> Settings or press CTRL+ALT+S.
The following window will open and check Show quick doc on mouse move
under IDE Settings -> Editor.
Or just press CTRL and hover your move over your method, class ...