Try to clarify design pattern.

[HansOlsson]

Closes #3242 Does this more clearly define that the design pattern is special and the rules don't apply in other cases?

[HansOlsson]

@casella do you agree with new text?

[casella]

As I wrote last November, I'm still doubtful how can users (and tools checking models) tell apart the cases (such as Modelica.Blocks) which are meant to be used with connect statements, from the case (such as Modelica.Media) where they aren't.