[DF][Doc][NFC] Add DescribeDataset to cheat sheet and release notes

README/ReleaseNotes/v626/index.md

@@ -54,6 +54,7 @@ The following people have contributed to this new version:
 
 - Add `Redefine` to the `RDataFrame` interface, which allows to overwrite the value of an existing column.
 - Add `Describe` to the `RDataFrame` interface, which allows to get useful information, e.g. the columns and their types.
+- Add `DescribeDataset` to the `RDataFrame` interface, which allows to get information about the dataset (subset of the output of Describe()).
 
 ## Histogram Libraries
 

tree/dataframe/src/RDataFrame.cxx

@@ -136,7 +136,7 @@ produce many different results in one event loop. Instant actions trigger the ev
 | GetNRuns() | Return the number of event loops run by this RDataFrame instance so far. |
 | GetNSlots() | Return the number of processing slots that RDataFrame will use during the event loop (i.e. the concurrency level). |
 | Describe() | Get useful information describing the dataframe, e.g. columns and their types. |
-
+| DescribeDataset() | Get useful information describing the dataset (subset of the output of Describe()). |
 
 
 \anchor introduction