[1.6.x] Fixed #21133 -- Clarifed documentation about strftime formatting.

Backport of 43a2ec7999 from master

docs/ref/class-based-views/mixins-date-based.txt

@@ -4,6 +4,11 @@ Date-based mixins
 
 .. currentmodule:: django.views.generic.dates
 
+.. note::
+    All the date formatting attributes in these mixins use
+    :func:`~time.strftime` format characters. Do not try to use the format
+    characters from the :ttag:`now` template tag as they are not compatible.
+
 YearMixin
 ---------
 

docs/ref/templates/builtins.txt

@@ -1245,6 +1245,10 @@ Formats a date according to the given format.
 Uses a similar format as PHP's ``date()`` function (http://php.net/date)
 with some differences.
 
+.. note::
+    These format characters are not used in Django outside of templates. They
+    were designed to be compatible with PHP to ease transitioning for designers.
+
 Available format strings:
 
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