Add sphinx doc for the sfaverage module.

Author: pavoljuhas

doc/manual/source/api/diffpy.srreal.rst

@@ -120,10 +120,19 @@ diffpy.srreal.scatteringfactortable module
 ------------------------------------------
 
 .. automodule:: diffpy.srreal.scatteringfactortable
-    :members:
+    :members: ScatteringFactorTable, SFTXray,
+      SFTElectron, SFTNeutron, SFTElectronNumber
     :undoc-members:
     :show-inheritance:
 
+diffpy.srreal.sfaverage module
+------------------------------
+
+.. automodule:: diffpy.srreal.sfaverage
+    :members:
+    :undoc-members:
+    :no-show-inheritance:
+
 diffpy.srreal.structureadapter module
 -------------------------------------