Update groupby section

Signed-off-by: Igoshev, Iaroslav <[email protected]>

docs/flow/modin/core/dataframe/algebra.rst

@@ -90,7 +90,14 @@ equals to the number of CPUs so that each single axis partition gets processed i
 GroupBy operator
 ----------------
 Evaluates GroupBy aggregation for that type of functions that can be executed via TreeReduce approach.
-To be able to form groups engine broadcasts ``by`` partitions to each partition of the source frame.
+To be able to form groups engine broadcasts ``by`` partitions to each partition of the source frame or
+applies range partitioning approach.
+
+This operator performs best when the cardinality of ``by`` columns is low (small number of output groups).
+At the ``Map`` stage, the operator computes the aggregation for each row partition individually, meaning,
+that the ``Reduce`` stage takes a dataframe with the following number of rows:
+``num_groups * n_row_parts``. If the number of groups is too high, there's a risk of getting a dataframe
+with even bigger than the initial shape at the ``Reduce`` stage.
 
 Default-to-pandas operator
 --------------------------