Using android Studio is very satisfying. However, there are sometimes bugs or glitches that can be very annoying. Within the last few days the internal (on hover) documentation like the JavaDoc was not shown properly. Its simply not displaying any information. In other words it seems like the documentation content part is empty or it is missing. I did not find out by myself what caused this but Stackoverflow was helping me .
To get ridged from this drawback you only have to adjust the height from the on hover window. But you should not be too overjoyed: It seems there is a bug resizing the window to the minimum with every restart.