Better document Deferred–thenable interactions

[gibson042]

(moved from jquery/jquery#3646 (comment) )

Update https://api.jquery.com/jQuery.Deferred/ and https://api.jquery.com/deferred.resolve/ to clearly define the behavior of directly resolving a Deferred with a thenable, and https://api.jquery.com/deferred.then/ to define the result of returning a thenable from .then().