adding note regarding return type of lambda CS-DSL

Author: Jolanrensen

core/generated-sources/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/ColumnsSelectionDsl.kt

@@ -40,9 +40,17 @@ public annotation class ColumnsSelectionDslMarker
  * (Any (combination of) [Access API][org.jetbrains.kotlinx.dataframe.documentation.AccessApi]).
  *
  * This DSL is initiated by a [Columns Selector][org.jetbrains.kotlinx.dataframe.ColumnsSelector] lambda,
- * which operates on the [Columns Selection DSL][org.jetbrains.kotlinx.dataframe.api.ColumnsSelectionDsl] and
- * expects you to return a [ColumnsResolver][org.jetbrains.kotlinx.dataframe.columns.ColumnsResolver]; an entity formed by calling any (combination) of the functions
+ * which operates in the context of the [Columns Selection DSL][org.jetbrains.kotlinx.dataframe.api.ColumnsSelectionDsl] and
+ * expects you to return a [SingleColumn][org.jetbrains.kotlinx.dataframe.columns.SingleColumn] or [ColumnSet][org.jetbrains.kotlinx.dataframe.columns.ColumnSet] (so, a [ColumnsResolver][org.jetbrains.kotlinx.dataframe.columns.ColumnsResolver]).
+ * This is an entity formed by calling any (combination) of the functions
  * in the DSL that is or can be resolved into one or more columns.
+ *
+ * #### NOTE:
+ * While you can use the [String API][org.jetbrains.kotlinx.dataframe.documentation.AccessApi.StringApi] and [KProperties API][org.jetbrains.kotlinx.dataframe.documentation.AccessApi.KPropertiesApi]
+ * in this DSL directly with any function, they are NOT valid return types for the
+ * [Columns Selector][org.jetbrains.kotlinx.dataframe.ColumnsSelector] lambda. You'd need to turn them into a [ColumnReference][org.jetbrains.kotlinx.dataframe.columns.ColumnReference] first, for instance
+ * with a function like [`col("name")`][org.jetbrains.kotlinx.dataframe.api.ColumnsSelectionDsl.col].
+ *
  * ### Check out: [Columns Selection DSL Grammar][org.jetbrains.kotlinx.dataframe.api.ColumnsSelectionDsl.DslGrammar]
  *
  *