chore: add docstring for agg

ibis/expr/operations/udf.py

@@ -351,3 +351,35 @@ class agg(_UDF):
     __slots__ = ()
 
     _base = AggUDF
+
+    @util.experimental
+    @classmethod
+    def builtin(cls, *args: Any, **kwargs: Any) -> Callable:
+        """Construct an aggregate user-defined function that is built-in to the backend.
+
+        Parameters
+        ----------
+        fn
+            The The function to wrap.
+        args
+            Configuration arguments for the UDF.
+        name
+            The name of the UDF in the backend if different from the function name.
+        schema
+            The schema in which the builtin function resides.
+        kwargs
+            Additional configuration arguments for the UDF.
+
+        Examples
+        --------
+        >>> import ibis
+        >>> ibis.options.interactive = True
+        >>> @ibis.udf.agg.builtin
+        ... def favg(a: float) -> float:
+        ...     '''Compute the average of a column using Kahan summation.'''
+        >>> t = ibis.examples.penguins.fetch()
+        >>> expr = favg(t.bill_length_mm)
+        >>> expr
+        43.9219298245614
+        """
+        return super().builtin(*args, **kwargs)