Fix CloudFront docs to match current reality.

CloudFront's create_distribution requires origin object. To match
the spirit of the docs, update docs to reflect creating the S3Origin
object first. Failure to do this results in to_xml() being called on your str, which breaks (exception thrown).

This might be worse than just assuming (and documenting) that passing a
string into origin for create_distribution implies
S3Origin(dns_name='thestr'). Design decision left for later.