bpo-44344: Document that pow can return a complex number for non-complex inputs. (GH-27853) (GH-29134)

Co-authored-by: Łukasz Langa <[email protected]>
(cherry picked from commit 887a557)

Co-authored-by: Mark Dickinson <[email protected]>

Author: miss-islington

Doc/library/functions.rst

@@ -1299,7 +1299,10 @@ are always available.  They are listed here in alphabetical order.
    operands, the result has the same type as the operands (after coercion)
    unless the second argument is negative; in that case, all arguments are
    converted to float and a float result is delivered.  For example, ``pow(10, 2)``
-   returns ``100``, but ``pow(10, -2)`` returns ``0.01``.
+   returns ``100``, but ``pow(10, -2)`` returns ``0.01``.  For a negative base of
+   type :class:`int` or :class:`float` and a non-integral exponent, a complex
+   result is delivered.  For example, ``pow(-9, 0.5)`` returns a value close
+   to ``3j``.
 
    For :class:`int` operands *base* and *exp*, if *mod* is present, *mod* must
    also be of integer type and *mod* must be nonzero. If *mod* is present and