(WIP) Clarify that interface/abstract method signatures do not need to match

Concrete method signatures can provide additional arguments provided they all have default values. See http://3v4l.org/dKFjC

(Unless I'm missing something) this seems like a big discrepancy between the docs/spec and the implementations.

Also, I would suggestion re-phrasing it slightly to...
`Concrete methods can enhance the required signature, as long as the concrete method includes the required  arguments & more - with each additional argument specifying a default value`