More references to the Implementation Notes in spec

[neilsjefferies]

Cross references encourage people to read the implementation notes since they clarify points and answer many questions.

[zimeon]

Editors' discussion at iPres: With v2 we plan to work with a tighter coupling of the specification and the implementation including links from the specification to the implementation notes at relevant places. This might also include moving some current notes into the implementation notes.

[rosy1280]

this feels like something that we would do at the end of V2, before we are releasing it as beta so that we can ensure more cross references to the implementation notes. we might also want to think about how this would be formatted as we're working on the spec.