[2.0.x] Fixed #28786 -- Doc'd middleware ordering considerations due to CommonMiddleware setting Content-Length.

Backport of bc95314ca6 from master

docs/ref/middleware.txt

@@ -488,7 +488,9 @@ Here are some hints about the ordering of various Django middleware classes:
 
 #. :class:`~django.middleware.common.CommonMiddleware`
 
-   Before any middleware that may change the response (it calculates ``ETags``).
+   Before any middleware that may change the response (it sets the ``ETag`` and
+   ``Content-Length`` headers). A middleware that appears before
+   ``CommonMiddleware`` and changes the response must reset the headers.
 
    After ``GZipMiddleware`` so it won't calculate an ``ETag`` header on gzipped
    contents.

docs/releases/1.11.txt

@@ -730,6 +730,12 @@ Miscellaneous
   ``Content-Length`` header as this is now done by
   :class:`~django.middleware.common.CommonMiddleware`.
 
+  If you have a middleware that modifies a response's content and appears
+  before ``CommonMiddleware`` in the ``MIDDLEWARE`` or ``MIDDLEWARE_CLASSES``
+  settings, you must reorder your middleware so that responses aren't modified
+  after ``Content-Length`` is set, or have the response modifying middleware
+  reset the ``Content-Length`` header.
+
 * :meth:`~django.apps.AppConfig.get_model` and
   :meth:`~django.apps.AppConfig.get_models` now raise
   :exc:`~django.core.exceptions.AppRegistryNotReady` if they're called before