Consider improving confusing behaviour for protected & private methods with Mock#responds_like etc

[James Mead]

• Should we retain the existing potentially confusing behaviour for protected & private methods?
  • If no, we'll need to come up with a deprecation strategy and migration path
  • If yes, it still might be worth trying to further improve the documentation, e.g. extra examples
  • Is there some crossover with #531 and/or #532 that we need to consider here?

See #580 & #583.