Update CumSum documentation for clarity and consistency

Refine the CumSum API documentation by improving phrasing, fixing formatting issues, and ensuring consistency in references. This improves readability and better explains behavior, especially for nullable values and default parameters.

Author: zaleslaw

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

@@ -24,31 +24,34 @@ import kotlin.reflect.typeOf
 /**
  * ## The CumSum Operation
  *
- * Computes the cumulative sum of the values in the {@get DATA_TYPE}.
+ * Computes the cumulative sums of the values in the each column from the {@get [CumSumDocs.DATA_TYPE]}
+ * from the first row to the last row.
  *
  * __NOTE:__ If the column contains nullable values and `skipNA` is set to `true`,
- * skips null values when computing the cumulative sum.
+ * null values are skipped when computing the cumulative sum.
  * Otherwise, any null value encountered will propagate null values in the output from that point onward.
  *
- * {@get [CUMSUM_PARAM] @param [columns]
+ * {@get [CumSumDocs.CUMSUM_PARAM] @param [columns\]
  * The names of the columns to apply cumSum operation.}
  *
- * @param [skipNA] Whether to skip null values (default: `true`).
+ * @param [skipNA\] Whether to skip null values (default: `true`).
  *
- * @return A new {@get DATA_TYPE} of the same type with the cumulative sum of the values.
+ * @return A new {@get [CumSumDocs.DATA_TYPE]} of the same type with the cumulative sums.
  *
- * {@get [CUMSUM_PARAM] @see [Selecting Columns][SelectSelectingOptions].}
+ * {@get [CumSumDocs.CUMSUM_PARAM] @see [Selecting Columns][SelectSelectingOptions].}
  * @see {@include [DocumentationUrls.CumSum]}
  */
 @ExcludeFromSources
 @Suppress("ClassName")
 private interface CumSumDocs {
     interface CUMSUM_PARAM
+
+    interface DATA_TYPE
 }
 
 /**
  * {@include [CumSumDocs]}
- * {@set DATA_TYPE [DataColumn]}.
+ * {@set [CumSumDocs.DATA_TYPE] [DataColumn]}
  * {@set [CumSumDocs.CUMSUM_PARAM]}
  */
 public fun <T : Number?> DataColumn<T>.cumSum(skipNA: Boolean = defaultCumSumSkipNA): DataColumn<T> =
@@ -114,7 +117,7 @@ private val supportedClasses = setOf(
 
 /**
  * {@include [CumSumDocs]}
- * {@set DATA_TYPE [DataFrame]}.
+ * {@set [CumSumDocs.DATA_TYPE] [DataFrame]}
  */
 public fun <T, C> DataFrame<T>.cumSum(
     skipNA: Boolean = defaultCumSumSkipNA,
@@ -124,14 +127,14 @@ public fun <T, C> DataFrame<T>.cumSum(
 
 /**
  * {@include [CumSumDocs]}
- * {@set DATA_TYPE [DataFrame]}.
+ * {@set [CumSumDocs.DATA_TYPE] [DataFrame]}
  */
 public fun <T> DataFrame<T>.cumSum(vararg columns: String, skipNA: Boolean = defaultCumSumSkipNA): DataFrame<T> =
     cumSum(skipNA) { columns.toColumnSet() }
 
 /**
  * {@include [CumSumDocs]}
- * {@set DATA_TYPE [DataFrame]}.
+ * {@set [CumSumDocs.DATA_TYPE] [DataFrame]}
  */
 @AccessApiOverload
 public fun <T> DataFrame<T>.cumSum(
@@ -141,15 +144,15 @@ public fun <T> DataFrame<T>.cumSum(
 
 /**
  * {@include [CumSumDocs]}
- * {@set DATA_TYPE [DataFrame]}.
+ * {@set [CumSumDocs.DATA_TYPE] [DataFrame]}
  */
 @AccessApiOverload
 public fun <T> DataFrame<T>.cumSum(vararg columns: KProperty<*>, skipNA: Boolean = defaultCumSumSkipNA): DataFrame<T> =
     cumSum(skipNA) { columns.toColumnSet() }
 
 /**
  * {@include [CumSumDocs]}
- * {@set DATA_TYPE [DataFrame]}.
+ * {@set [CumSumDocs.DATA_TYPE] [DataFrame]}
  * {@set [CumSumDocs.CUMSUM_PARAM]}
  */
 public fun <T> DataFrame<T>.cumSum(skipNA: Boolean = defaultCumSumSkipNA): DataFrame<T> =
@@ -163,7 +166,7 @@ public fun <T> DataFrame<T>.cumSum(skipNA: Boolean = defaultCumSumSkipNA): DataF
 
 /**
  * {@include [CumSumDocs]}
- * {@set DATA_TYPE [GroupBy]}.
+ * {@set [CumSumDocs.DATA_TYPE] [GroupBy]}
  */
 public fun <T, G, C> GroupBy<T, G>.cumSum(
     skipNA: Boolean = defaultCumSumSkipNA,
@@ -172,14 +175,14 @@ public fun <T, G, C> GroupBy<T, G>.cumSum(
 
 /**
  * {@include [CumSumDocs]}
- * {@set DATA_TYPE [GroupBy]}.
+ * {@set [CumSumDocs.DATA_TYPE] [GroupBy]}
  */
 public fun <T, G> GroupBy<T, G>.cumSum(vararg columns: String, skipNA: Boolean = defaultCumSumSkipNA): GroupBy<T, G> =
     cumSum(skipNA) { columns.toColumnSet() }
 
 /**
  * {@include [CumSumDocs]}
- * {@set DATA_TYPE [GroupBy]}.
+ * {@set [CumSumDocs.DATA_TYPE] [GroupBy]}
  */
 @AccessApiOverload
 public fun <T, G> GroupBy<T, G>.cumSum(
@@ -189,7 +192,7 @@ public fun <T, G> GroupBy<T, G>.cumSum(
 
 /**
  * {@include [CumSumDocs]}
- * {@set DATA_TYPE [GroupBy]}.
+ * {@set [CumSumDocs.DATA_TYPE] [GroupBy]}
  */
 @AccessApiOverload
 public fun <T, G> GroupBy<T, G>.cumSum(
@@ -199,7 +202,7 @@ public fun <T, G> GroupBy<T, G>.cumSum(
 
 /**
  * {@include [CumSumDocs]}
- * {@set DATA_TYPE [GroupBy]}.
+ * {@set [CumSumDocs.DATA_TYPE] [GroupBy]}
  * {@set [CumSumDocs.CUMSUM_PARAM]}
  */
 public fun <T, G> GroupBy<T, G>.cumSum(skipNA: Boolean = defaultCumSumSkipNA): GroupBy<T, G> =