Improved error message for ASCIIUsernameValidator.

Follow up to 10bb21e71e3668f770493e2af0d0e0271830f7a3.

Co-authored-by: Paolo Melchiorre <paolo@melchiorre.org>

django/contrib/auth/validators.py

@@ -9,8 +9,8 @@ from django.utils.translation import gettext_lazy as _
 class ASCIIUsernameValidator(validators.RegexValidator):
     regex = r"^[\w.@+-]+\Z"
     message = _(
-        "Enter a valid username. This value may contain only ASCII letters, "
-        "numbers, and @/./+/-/_ characters."
+        "Enter a valid username. This value may contain only unaccented lowercase a-z "
+        "and uppercase A-Z letters, numbers, and @/./+/-/_ characters."
     )
     flags = re.ASCII