Added note about password updates on argon2 attributes change.

docs/topics/auth/passwords.txt

@@ -224,8 +224,8 @@ However, Django can only upgrade passwords that use algorithms mentioned in
 :setting:`PASSWORD_HASHERS`, so as you upgrade to new systems you should make
 sure never to *remove* entries from this list. If you do, users using
 unmentioned algorithms won't be able to upgrade. Hashed passwords will be
-updated when increasing (or decreasing) the number of PBKDF2 iterations or
-bcrypt rounds.
+updated when increasing (or decreasing) the number of PBKDF2 iterations, bcrypt
+rounds, or argon2 attributes.
 
 Be aware that if all the passwords in your database aren't encoded in the
 default hasher's algorithm, you may be vulnerable to a user enumeration timing