Recommending <picture> for responsive image might be harmful

Hi, just discovered Performance Kit, and as a responsive images addict, I noticed you present the syntax with `<picture>` as "a standard responsive image".

This is not the recommended syntax for most responsive images that don't need Art Direction.

Jason Grigsby wrote about this a few year ago:
https://cloudfour.com/thinks/dont-use-picture-most-of-the-time/
