[2.2.x] Fixed #15742 -- Fixed an example of collecting selected objects in ModelAdmin.actions docs.

The queryset argument is already filtered, and request.POST doesn't
contain all selected objects when "Select All" is used.

Backport of e651b3095c950627b1eed1527f2bb011ddad03de from master

docs/ref/contrib/admin/actions.txt

@@ -238,14 +238,16 @@ you'd want to let the user choose a format, and possibly a list of fields to
 include in the export. The best thing to do would be to write a small action
 that simply redirects to your custom export view::
 
-    from django.contrib import admin
     from django.contrib.contenttypes.models import ContentType
     from django.http import HttpResponseRedirect
 
     def export_selected_objects(modeladmin, request, queryset):
-        selected = request.POST.getlist(admin.ACTION_CHECKBOX_NAME)
+        selected = queryset.values_list('pk', flat=True)
         ct = ContentType.objects.get_for_model(queryset.model)
-        return HttpResponseRedirect("/export/?ct=%s&ids=%s" % (ct.pk, ",".join(selected)))
+        return HttpResponseRedirect('/export/?ct=%s&ids=%s' % (
+            ct.pk,
+            ','.join(str(pk) for pk in selected),
+        ))
 
 As you can see, the action is the simple part; all the complex logic would
 belong in your export view. This would need to deal with objects of any type,