Explained why UserCreationForm performs custom validation of usernames. Refs #13147.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17225 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/contrib/auth/forms.py

@@ -69,6 +69,8 @@ class UserCreationForm(forms.ModelForm):
         fields = ("username",)
 
     def clean_username(self):
+        # Since User.username is unique, this check is redundant,
+        # but it sets a nicer error message than the ORM. See #13147.
         username = self.cleaned_data["username"]
         try:
             User.objects.get(username=username)