Backport 5a574e8

Improve Fl_Button event handling documentation (#1267)

src/Fl_Button.cxx

@@ -254,10 +254,19 @@ void Fl_Button::key_release_timeout(void *d)
 
  Derived classes may handle this differently.
 
- A button may request callbacks with \p when() \p FL_WHEN_CHANGED,
- \p FL_WHEN_NOT_CHANGED, and \p FL_WHEN_RELEASE, triggering the callback
- reasons \p FL_REASON_CHANGED, \p FL_REASON_SELECTED,
- and \p FL_REASON_DESELECTED.
+ Calling `when()` will tell the button widget when to call the callback.
+
+ Setting `FL_WHEN_RELEASE` will call the callback only if the button value
+ changed. It's called during `FL_RELEASE` and `FL_KEYBOARD` events with
+ `FL_REASON_RELEASED` set as the callback reason.
+
+ Setting `FL_WHEN_CHANGED` will call the callback with `FL_REASON_CHANGED`
+ every time the value of the button changes during `FL_DRAG`, `FL_RELEASE`,
+ and `FL_KEYBOARD` events.
+
+ Setting `FL_WHEN_NOT_CHANGED` will trigger a callback during `FL_RELEASE`
+ events, even if the value of the button die *not* change. For radio buttons,
+ this is also true during `FL_KEYBOARD` events.
 
  \param[in] X, Y, W, H position and size of the widget
  \param[in] L widget label, default is no label