[doc] add dummy documentation for roi selection

doc/index.rst

@@ -39,5 +39,6 @@ This is the list of actual widgets/boxes proposed by the workflow tool:
   widgets/pymca_ft.rst
   widgets/pymca_k_weight.rst
   widgets/pymca_normalization.rst
+  widgets/xas_roi.rst
   widgets/xas_input.rst
   widgets/xas_output.rst

doc/widgets/xas_roi.rst

+ROI definition
+==============
+
+Use to define a roi on the dataset.
+
+.. image:: img/xas_roi.png

orangecontrib/xas/widgets/roi.py

@@ -46,7 +46,7 @@ class RoiSelectionOW(OWWidget):
     Dataset.
     """
 
-    name = "ROI selection"
+    name = "ROI definition"
     id = "orange.widgets.id06workflow.roiselection"
     description = "Select data Region Of Interest"
     icon = "icons/image-select-box.svg"