fix 154 by documenting .cast
incorporate first pass feedback from @stefanpenner
change wording to -The resulting promise-
update .cast docs with resolve comparison info from @domenic

@domenic added notes in domenic/promises-unwrapping#86
those notes comparing promise.cast with promise.resolve have been
added to .cast docs
add @domenics latest suggestions

RSVP.promise -> RSVP.promise.cast
set then to code
additional text for clarification
Merge pull request #159 from mikepmunroe/154/document-cast

fix 154 by documenting .cast