Fixed #18023 -- Documented simplejson issues.

Thanks Luke Plant for reporting the issue and Alex Ogier for thoroughly
investigating it.

docs/releases/1.5.txt

@@ -172,6 +172,54 @@ If you were using the ``data`` parameter in a PUT request without a
 ``content_type``, you must encode your data before passing it to the test
 client and set the ``content_type`` argument.
 
+.. _simplejson-incompatibilities:
+
+System version of :mod:`simplejson` no longer used
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+:ref:`As explained below <simplejson-deprecation>`, Django 1.5 deprecates
+:mod:`django.utils.simplejson` in favor of Python 2.6's built-in :mod:`json`
+module. In theory, this change is harmless. Unfortunately, because of
+incompatibilities between versions of :mod:`simplejson`, it may trigger errors
+in some circumstances.
+
+JSON-related features in Django 1.4 always used :mod:`django.utils.simplejson`.
+This module was actually:
+
+- A system version of :mod:`simplejson`, if one was available (ie. ``import
+  simplejson`` works), if it was more recent than Django's built-in copy or it
+  had the C speedups, or
+- The :mod:`json` module from the standard library, if it was available (ie.
+  Python 2.6 or greater), or
+- A built-in copy of version 2.0.7 of :mod:`simplejson`.
+
+In Django 1.5, those features use Python's :mod:`json` module, which is based
+on version 2.0.9 of :mod:`simplejson`.
+
+There are no known incompatibilities between Django's copy of version 2.0.7 and
+Python's copy of version 2.0.9. However, there are some incompatibilities
+between other versions of :mod:`simplejson`:
+
+- While the :mod:`simplejson` API is documented as always returning unicode
+  strings, the optional C implementation can return a byte string. This was
+  fixed in Python 2.7.
+- :class:`simplejson.JSONEncoder` gained a ``namedtuple_as_object`` keyword
+  argument in version 2.2.
+
+More information on these incompatibilities is available in `ticket #18023`_.
+
+The net result is that, if you have installed :mod:`simplejson` and your code
+uses Django's serialization internals directly -- for instance
+:class:`django.core.serializers.json.DjangoJSONEncoder`, the switch from
+:mod:`simplejson` to :mod:`json` could break your code. (In general, changes to
+internals aren't documented; we're making an exception here.)
+
+At this point, the maintainers of Django believe that using :mod:`json` from
+the standard library offers the strongest guarantee of backwards-compatibility.
+They recommend to use it from now on.
+
+.. _ticket #18023: https://code.djangoproject.com/ticket/18023#comment:10
+
 String types of hasher method parameters
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -282,13 +330,21 @@ Miscellaneous
 Features deprecated in 1.5
 ==========================
 
+.. _simplejson-deprecation:
+
 ``django.utils.simplejson``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Since Django 1.5 drops support for Python 2.5, we can now rely on the
-:mod:`json` module being in Python's standard library -- so we've removed
-our own copy of ``simplejson``. You can safely change any use of
-:mod:`django.utils.simplejson` to :mod:`json`.
+:mod:`json` module being available in Python's standard library, so we've
+removed our own copy of :mod:`simplejson`. You should now import :mod:`json`
+instead :mod:`django.utils.simplejson`.
+
+Unfortunately, this change might have unwanted side-effects, because of
+incompatibilities between versions of :mod:`simplejson` -- see the
+:ref:`backwards-incompatible changes <simplejson-incompatibilities>` section.
+If you rely on features added to :mod:`simplejson` after it became Python's
+:mod:`json`, you should import :mod:`simplejson` explicitly.
 
 ``itercompat.product``
 ~~~~~~~~~~~~~~~~~~~~~~