Add examples for Regex-related Split operations

Author: koperagen

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

@@ -99,6 +99,21 @@ public fun <T, C> Split<T, C>.by(
         }
     }
 
+/**
+ * Example:
+ * ```
+ * dataFrameOf("str" to listOf("1    2 3     4"))
+ *   .split("str").by("\\s+".toRegex())
+ *   // when the list of explicitly specified columnNames is not long enough (or none at all),
+ *   // names for additional columns are generates
+ *   .into()
+ * ```
+ * Result:
+ * ```
+ *    split1 split2 split3 split4
+ *         1      2      3      4
+ * ```
+ */
 @Interpretable("ByRegex")
 public fun <T, C> Split<T, C>.by(
     regex: Regex,
@@ -138,7 +153,13 @@ internal inline fun <T, C, R> Split<T, C>.by(
 // region match
 
 /**
- * Creates new String columns according to MatchResult [capturing groups](https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/-match-result/group-values.html)
+ * Creates new String columns according to MatchResult [capturing groups](https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/-match-result/group-values.html),
+ * excluding the first group which is entire matched String.
+ * Example:
+ * ```
+ * dataFrameOf("str" to listOf("100 ml", "1 L"))
+ *      .split { "str"<String>() }.match("(\\d+)\\s*(ml|l|L)").into("volume", "unit")
+ * ```
  * Created columns will be nullable if [regex] doesn't match some rows or there are nulls in original column
  * Check [Split.by] overload with regex parameter if you're looking to split String value by [Regex] delimiter
  */