Adding documentation

Author: Petr Jacka

README/ReleaseNotes/v626/index.md

@@ -146,6 +146,7 @@ If you have been trying RNTuple for a while, these are the other important chang
 - Add [`DescribeDataset`](https://root.cern/doc/master/classROOT_1_1RDF_1_1RInterface.html#a1bc5b86a2a834bb06711fb535451146d) to the `RDataFrame` interface, which allows to get information about the dataset (subset of the output of Describe()).
 - Add [`DefinePerSample`](https://root.cern/doc/master/classROOT_1_1RDF_1_1RInterface.html#a29d77593e95c0f84e359a802e6836a0e), a method which makes it possible to define columns based on the sample and entry range being processed. It is also a useful way to register callbacks that should only be called when the input dataset/TTree changes.
 - Add [`HistoND`](https://root.cern/doc/master/classROOT_1_1RDF_1_1RInterface.html#a0c9956a0f48c26f8e4294e17376c7fea) action that fills a N-dimensional histogram.
+- Add [`HistoNSparseD`](https://root.cern/doc/master/classROOT_1_1RDF_1_1RInterface.html#a5f3e2f0a3d1c8e4f0e2f3e7f0e8c6b7a) action that fills a sparse N-dimensional histogram.
 - `Book` now supports just-in-time compilation, i.e. it can be called without passing the column types as template parameters (with some performance penalty, as usual).
 - As an aid to `RDataSource` implementations with which collection sizes can be retrieved more efficiently than the full collection, `#var` can now be used as a short-hand notation for column name `R_rdf_sizeof_var`.
 - Helpers have been added to export data from `RDataFrame` to RooFit datasets. See the "RooFit Libraries" section below for more details, or see [the tutorial](https://root.cern/doc/master/rf408__RDataFrameToRooFit_8C.html).
@@ -189,6 +190,7 @@ Other notable additions and improvements include:
 - Add support for the following operations:
   - `DefinePerSample`
   - `HistoND`
+  - `HistoNSparseD`
   - `Redefine`
 - Make sure a user-provided `npartitions` parameter to a distributed RDataFrame constructor always takes precedence over the value computed by default.
 - Improve support for friend trees in distributed executions, now any kind of friendship layout between the main tree and the friend tree(s) is expected to work.

tree/dataframe/inc/ROOT/RDF/RMergeableValue.hxx

@@ -245,6 +245,7 @@ actions:
 - [Histo{1D,2D,3D}]
   (classROOT_1_1RDF_1_1RInterface.html#a247ca3aeb7ce5b95015b7fae72983055)
 - [HistoND](classROOT_1_1RDF_1_1RInterface.html#a0c9956a0f48c26f8e4294e17376c7fea)
+- [HistoNSparseD](classROOT_1_1RDF_1_1RInterface.html#a5f3e2f0a3d1c8e4f0e2f3e7f0e8c6b7a)
 - [Profile{1D,2D}]
   (classROOT_1_1RDF_1_1RInterface.html#a8ef7dc16b0e9f7bc9cfbe2d9e5de0cef)
 - [Stats](classROOT_1_1RDF_1_1RInterface.html#abc68922c464e472f5f856e8981955af6)

tree/dataframe/src/RDataFrame.cxx

@@ -135,6 +135,7 @@ produce many different results in one event loop. Instant actions trigger the ev
 | GraphAsymmErrors() | Fills a TGraphAsymmErrors. Should be used for any type of graph with errors, including cases with errors on one of the axes only. If multi-threading is enabled, the order of the points may not be the one expected, it is therefore suggested to sort if before drawing. |
 | Histo1D(), Histo2D(), Histo3D() | Fill a one-, two-, three-dimensional histogram with the processed column values. |
 | HistoND() | Fill an N-dimensional histogram with the processed column values. |
+| HistoNSparseD() | Fill an N-dimensional sparse histogram with the processed column values. Memory is allocated only for non-empty bins. |
 | Max() | Return the maximum of processed column values. If the type of the column is inferred, the return type is `double`, the type of the column otherwise.|
 | Mean() | Return the mean of processed column values.|
 | Min() | Return the minimum of processed column values. If the type of the column is inferred, the return type is `double`, the type of the column otherwise.|
@@ -737,7 +738,7 @@ parts of the RDataFrame API currently work with this package. The subset that is
 - FilterMissing
 - Graph
 - Histo[1,2,3]D
-- HistoND
+- HistoND, HistoNSparseD
 - Max
 - Mean
 - Min