Move the normative bits of the picture spec to the HTML spec.

* Introduction is missing.
* Interaction with the Preload Scanner is missing.
* Specified what picture and source "represent".
* Made <img srcset> have the same authoring conformance requirement as
  <source srcset> about not omitting whitespace when omitting descriptors.
* Commented out some examples.
* Uncommented some picture parent checks in some algorithms.
* Moved a note and added a note.
* Editorial tweaks.