Fix documentation for #781

[fmerges]

This fixes the documentation. But, on the same topic, currently the extension doesn't really support plaintext anymore, and I guess it's not encouraged per se:

https://github.com/mattupstate/flask-security/blob/89198288bc416a7921b4973d8e897993942bb428/flask_security/utils.py#L120-L127

So why not eliminate it completely? Additionally, the transparent update of plaintext passwords to the configured hash is also something arguable (backward compatibility?)

[jirikuncar]

The plaintext passwords should still be possible: https://github.com/mattupstate/flask-security/blob/89198288bc416a7921b4973d8e897993942bb428/flask_security/utils.py#L455