Fixed #5341 -- Clarified docs/db-api.txt docs about params in extra(). Thanks, Paul Smith

git-svn-id: http://code.djangoproject.com/svn/django/trunk@6163 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/db-api.txt

@@ -799,6 +799,9 @@ of the arguments is required, but you should use at least one of them.
 
         Entry.objects.extra(where=['headline=%s'], params=['Lennon'])
 
+    The combined number of placeholders in the list of strings for ``select``
+    or ``where`` should equal the number of values in the ``params`` list.
+
 QuerySet methods that do not return QuerySets
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