Mention @gzip_page explicitly as a reason why the toolbar doesn't show up (#2392)

Closes #2375.

Author: matthiask

docs/tips.rst

@@ -16,6 +16,10 @@ requests and return responses. Putting the debug toolbar middleware *after* the
 ``FlatpageFallbackMiddleware`` middleware, for example, means the toolbar will
 not show up on flatpages.
 
+The response must also not be compressed;
+:func:`~django.views.decorators.gzip.gzip_page` and ``GZipMiddleware`` listed
+after the toolbar in ``MIDDLEWARE`` are common causes.
+
 Check your browser's caching
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