groupBy kdocs fixes

Author: AndreiKingsley

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/groupBy.kt

@@ -542,7 +542,7 @@ public interface GroupBy<out T, out G> : Grouped<G> {
      * a row from [keys] and its corresponding group of rows (as [DataFrame]).
      *
      * If [groupedColumnName] is provided, the groups will be stored
-     * in a [FrameColumn] with that name; otherwise, a default name is used.
+     * in a [FrameColumn] with that name; otherwise, a default name "group" is used.
      *
      * @param groupedColumnName The name of the column in which to store grouped data;
      * if `null`, a default name will be used.

docs/StardustDocs/topics/groupBy.md

@@ -142,11 +142,7 @@ df.groupBy { name }.concat()
 <inline-frame src="resources/org.jetbrains.kotlinx.dataframe.samples.api.Modify.concatGroupBy.html" width="100%"/>
 <!---END-->
 
-## Reducing
-
-TODO
-
-## Aggregation
+<!---TODO ## Reducing--->
 
 To compute one or several [statistics](summaryStatistics.md) per every group of `GroupBy` use `aggregate` function. 
 Its body will be executed for every data group and has a receiver of type [`DataFrame`](DataFrame.md) that represents current data group being aggregated.