[TDF] Add the keyword "cursor" to the TDataSource documentation

it's a very clear and general concept which applies perfectly in the
context of the TDataSource. Thanks to J. Blomer for mentioning it
in the first place!

tree/treeplayer/inc/ROOT/TDataSource.hxx

@@ -26,7 +26,7 @@ namespace TDF {
 \ingroup dataframe
 \brief The TDataSource interface dictates how a TDataFrame interface to an arbitrary data format should look like.
 
-A TDataSource allows to seamlessly provide an adaptor for any kind of data set or data format to the TDataFrame.
+A TDataSource allows to seamlessly provide an adaptor for any kind of data set or data format to the TDataFrame. Another way to imagine it is a veritable "cursor".
 */
 class TDataSource {
 public: