[1.10.x] Doc'd the need to provide initial for formset submissions.

Backport of 80f5a4d87a from master

docs/topics/forms/formsets.txt

@@ -80,6 +80,11 @@ There are now a total of three forms showing above. One for the initial data
 that was passed in and two extra forms. Also note that we are passing in a
 list of dictionaries as the initial data.
 
+If you use an ``initial`` for displaying a formset, you should pass the same
+``initial`` when processing that formset's submission so that the formset can
+detect which forms were changed by the user. For example, you might have
+something like: ``ArticleFormSet(request.POST, initial=[...])``.
+
 .. seealso::
 
     :ref:`Creating formsets from models with model formsets <model-formsets>`.