docs(python): improve docstring of (Expr|Series).map_elements (#16079)

py-polars/polars/expr/expr.py

@@ -4866,6 +4866,12 @@ def map_elements(
               `pl.col("col_name").sqrt()`.
             - For mapping inner elements of lists, consider:
               `pl.col("col_name").list.eval(pl.element().sqrt())`.
+            - For mapping elements of struct fields, consider:
+              `pl.col("col_name").struct.field("field_name").sqrt()`.
+
+            If you want to replace the original column or field,
+            consider :meth:`.with_columns <polars.DataFrame.with_columns>`
+            and :meth:`.with_fields <polars.Expr.struct.with_fields>`.
 
         The UDF is applied to each element of a column. Note that, in a GroupBy
         context, the column will have been pre-aggregated and so each element

py-polars/polars/series/series.py

@@ -5451,6 +5451,8 @@ def map_elements(
             - For mapping elements of a series, consider: `s.sqrt()`.
             - For mapping inner elements of lists, consider:
               `s.list.eval(pl.element().sqrt())`.
+            - For mapping elements of struct fields, consider:
+              `s.struct.field("field_name").sqrt()`.
 
         If the function returns a different datatype, the return_dtype arg should
         be set, otherwise the method will fail.