Do you use a wide screen? Or have you noticed that the documentation window hides the text in the editor at the moment when you want to see one or two lines above the caret? A few days ago I was discussing with the editor guys, why NetBeans displays the documentation window above the code completion window. As a result there is issue #144362, which was resolved at the beginning of this month.
If you switch on the Display Documetation Window Next to Completion property in the Tools -> Options -> Editor category, the documentation window is displayed on the left or right of the code completion window.