comment on capitalization

Author: volodymyrss

astroquery/heasarc/core.py

@@ -297,7 +297,12 @@ def _args_to_payload(self, **kwargs):
         action : str, optional
             Type of action to be taken (defaults to 'Query')
         """
-        # Define the basic query for this object
+        # All user-facing parameters are lower case, for consistency and to preserve current interface.
+        # The parameters as passed to the HEASARC service are capitalized according to the HEASARC requirements.
+        # It means that in some cases, unfortunately, this module will identical but differently capitalized, e.g. `Entry` vs `entry`.
+        # All transformations to the HEASARC-specific capitalization is done in this `_args_to_payload` function.
+
+        # Define the basic query for this object        
 
         mission = kwargs.pop('mission')