Doubleclicking
the Straighten tool in the toolbox (or pressing the Enter
key when the tool is selected) will bring up its preferences dialog:
Straighten Preferences
The tolerance for the cursor when snapping to existing paths, in pixels.
When enabled, the constrain angle is ignored when straightening the segment (i.e., it will always be parallel to an artboard edge).
Shows a brief description of each preference control when the cursor is being hovered over it.
Opens the help documentation in the Astute Manager. If this does not automatically appear, please ensure your Astute Manager is running first.