[1.6.x] Added docs for the hasher's iteration count changes.

Backport of 28b70425af from master.

docs/releases/1.6.txt

@@ -369,7 +369,8 @@ Minor features
   increased by 20%. This backwards compatible change will not affect
   existing passwords or users who have subclassed
   ``django.contrib.auth.hashers.PBKDF2PasswordHasher`` to change the
-  default value.
+  default value. Passwords :ref:`will be upgraded <password-upgrades>` to use
+  the new iteration count as necessary.
 
 Backwards incompatible changes in 1.6
 =====================================

docs/topics/auth/passwords.txt

@@ -124,6 +124,8 @@ algorithm.
    output)``. For example:
    ``bcrypt$$2a$12$NT0I31Sa7ihGEWpka9ASYrEFkhuTNeBQ2xfZskIiiJeyFXhRgS.Sy``.
 
+.. _increasing-password-algorithm-work-factor:
+
 Increasing the work factor
 --------------------------
 
@@ -167,6 +169,8 @@ default PBKDF2 algorithm:
 That's it -- now your Django install will use more iterations when it
 stores passwords using PBKDF2.
 
+.. _password-upgrades:
+
 Password upgrading
 ------------------
 
@@ -181,6 +185,10 @@ However, Django can only upgrade passwords that use algorithms mentioned in
 sure never to *remove* entries from this list. If you do, users using un-
 mentioned algorithms won't be able to upgrade.
 
+.. versionadded:: 1.6
+
+    Passwords will be upgraded when changing the PBKDF2 iteration count.
+
 .. _sha1: http://en.wikipedia.org/wiki/SHA1
 .. _pbkdf2: http://en.wikipedia.org/wiki/PBKDF2
 .. _nist: http://csrc.nist.gov/publications/nistpubs/800-132/nist-sp800-132.pdf