[DF][NFC] Mention AsRNode in the progress bar documentation

fixes #15040

tree/dataframe/src/RDataFrame.cxx

@@ -1512,7 +1512,8 @@ ROOT::RDF::Experimental::AddProgressBar(df);
 
 Alternatively, RDataFrame can be cast to an RNode first, giving the user more flexibility 
 For example, it can be called at any computational node, such as Filter or Define, not only the head node,
-with no change to the ProgressBar function itself: 
+with no change to the ProgressBar function itself (please see the [Efficient analysis in Python](#python) 
+section for appropriate usage in Python): 
 ~~~{.cpp}
 ROOT::RDataFrame df("tree", "file.root");
 auto df_1 = ROOT::RDF::RNode(df.Filter("x>1"));