Document the unusability metadata of password

[Flimm]

This is a small patch to documentation.

It isn't clear how set_unusable_password marks a user's password as unusable. This commit clarifies that the password field itself contains metadata marking a password as unusable.

Trac ticket number

N/A

[jaap3]

I agree that this is an important distinction.

has_usable_password can also use some clarification. It strictly looks at the UNUSABLE_PASSWORD_PREFIX (and None) to determine that a password is usable. It considers a blank string to be usable. This is currently implied, but requires the reader to understand wat set_unusable_password does in the first place.