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.

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.
Merge pull request #1546 from emidln/develop

Fix CloudFront docs to match current reality.