For module histogram
"The histogram is directly linked to the <emphasis>exposure</emphasis> module "
"described in <xref linkend=\"exposure\"/>, and you can operate some of the "
"exposure module's controls from the histogram. You can left-click towards "
"the right hand side of the histogram and then drag right to increase or drag "
"left to decrease the exposure. In a similar manner you can control the black "
"level by clicking and dragging in the left hand side."
I suggest to add <<doubleclick resets>> or some thing like that (That is a feature of histogram module)
For module crop and rotate
"First off, select what aspect ratio you want and size the crop boundaries by "
"dragging border and corner handles. Use the button right of the aspect box, "
"to swap between portrait and landscape mode. You can move around the crop "
"rectangle by holding down left mouse button and move around. When you are "
"done and want to execute the crop, just give focus to another module. You "
"can at any time change your crop area by just revisiting this module."
I think we can also execute the crop by doubleclicking.