[DOCS] Ember.RSVP.cast is not documented

[chadhietala]

No description provided.

[rwjblue]

RSVP.cast is just Ember.RSVP.resolve AFAIK (see here). I think Ember.RSVP.Promise.resolve would be preferred over either, but @stefanpenner should chime in to confirm/deny...

[stefanpenner]

cast is dead, don't use it, as 100% of cast usages can be expressed as Promise.resolve. It exists purely as a deprecated compat thing.

It was part of early days of the spec, Promise.cast and Promise.resolve where two different algorithms. But it was decided, that in all cases we would prefer Promise.cast algorithm but using the Promise.resolve name.