Fixed #7609 -- Noted in the model fields reference documentation that…

Fixed #7609 -- Noted in the model fields reference documentation that `PositiveIntegerField` accepts the value 0 for backwards-compatibility reasons. Thanks to everyone involved in the resolution of this issue, including Paul Collins for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16972 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/ref/models/fields.txt

@@ -805,7 +805,8 @@ this as a ``<select>`` box with "Unknown", "Yes" and "No" choices.
 
 .. class:: PositiveIntegerField([**options])
 
-Like an :class:`IntegerField`, but must be positive.
+Like an :class:`IntegerField`, but must be either positive or zero (`0`).
+The value `0` is accepted for backward compatibility reasons.
 
 ``PositiveSmallIntegerField``
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