add docstring to GroupByImpl class

Author: koperagen

core/generated-sources/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/GroupByImpl.kt

@@ -20,6 +20,9 @@ import org.jetbrains.kotlinx.dataframe.ncol
 import org.jetbrains.kotlinx.dataframe.nrow
 import org.jetbrains.kotlinx.dataframe.values
 
+/**
+ * @property df DataFrame containing [groups] column and key columns. Represents GroupBy.
+ */
 internal class GroupByImpl<T, G>(
     val df: DataFrame<T>,
     override val groups: FrameColumn<G>,

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/GroupByImpl.kt

@@ -20,6 +20,9 @@ import org.jetbrains.kotlinx.dataframe.ncol
 import org.jetbrains.kotlinx.dataframe.nrow
 import org.jetbrains.kotlinx.dataframe.values
 
+/**
+ * @property df DataFrame containing [groups] column and key columns. Represents GroupBy.
+ */
 internal class GroupByImpl<T, G>(
     val df: DataFrame<T>,
     override val groups: FrameColumn<G>,