Throughout the documentations, we are using 'encrypt' incorrectly.
Encrypt means that someone will eventually decrypt the message,
which is obviously not the case for Devise.
I'm changing the docs to use 'hashing' instead.
However, I left the database field as `encrypted_password` for now.
I'll update the db field in an upcoming PR.
This method is part of the protected API and is used by custom
encryption engines (like `devise-encryptable`) to hook the custom
encryption logic in the models.
Fixes#2730
Pat Allan