doc/SASL-SCRAM: clarify setup instructions

doc/SASL-SCRAM: clarify setup instructions

- Explain the ./configure argument to force GNU libidn to be available
  and what to look for when it prints its configuration.

- Reorder mechanisms in order of strength when asking to decide.

- Clarify that regular PBKDF2 credentials definitely allow impersonation,
  and that this is why the SCRAM module does nothing if this style of
  credentials is being used.

- Move loadmodule advice to next to eachother.

- Space everything out a bit more for readability.

- Directly discourage uncommenting the SCRAM loadmodule line in the
  example configuration file without having read the documentation.