Enhance: edit doc py::native_enum feature in upgrade.rst (#5885)

* Enhance: edit doc py::native_enum feature in upgrade.rst

Added information about the inclusion requirement for py::native_enum feature.

* [skip ci] Polish wording

---------

Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>

Author: rangsimanketkaew

docs/upgrade.rst

@@ -48,6 +48,8 @@ C++ enumerations as native Python types — typically standard-library
 ``enum.Enum`` or related subclasses. This provides improved integration with
 Python's enum system, compared to the older (now deprecated) ``py::enum_``.
 See `#5555 <https://github.com/pybind/pybind11/pull/5555>`_ for details.
+Note that ``#include <pybind11/native_enum.h>`` is not included automatically
+and must be added explicitly.
 
 Functions exposed with pybind11 are now pickleable. This removes a
 long-standing obstacle when using pybind11-bound functions with Python features