[1.6.x] Force update of the password on iteration count changes.

Backport of 7d0d0dbf26a3c0d16e9c2b930fd6d7b89f215946 from master.
[1.6.x] Added docs for the hasher's iteration count changes.

Backport of 28b70425afb2fb8bcbec09d249e37fa786f8a155 from master.