fix group grammar

Author: AndreiKingsley

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

@@ -67,7 +67,11 @@ internal interface GroupDocs {
      * __`.`__[**`into`**][GroupClause.into]**`(`**`groupName: `[`String`][String]**`)`**
      *
      * {@include [Indent]}
-     * __`.`__[**`into`**][GroupClause.into]**`  {  `**`groupNameExpression: `[`ColumnsSelector`][ColumnsSelector]**`  }  `**
+     * __`.`__[**`into`**][GroupClause.into]` { column: `[`ColumnsSelectionDsl<T>`][ColumnsSelectionDsl]`.(`[`ColumnWithPath<C>`][ColumnWithPath]`) -> `[`String`][String]` }`
+     *
+     * {@include [Indent]}
+     * __`.`__[**`into`**][GroupClause.into]` { column: `[`ColumnsSelectionDsl<T>`][ColumnsSelectionDsl]`.(`[`ColumnWithPath<C>`][ColumnWithPath]`) -> `[`AnyColumnReference`][AnyColumnReference]` }`
+     *
      */
     interface Grammar
 }
@@ -156,6 +160,7 @@ public class GroupClause<T, C>(internal val df: DataFrame<T>, internal val colum
  *
  * ### Example:
  * ```kotlin
+ * // For each selected column, place it under its under a group with its type as name (individual for each column):
  * df.group { all() }.into { it.type().toString() }
  * ```
  *