Specify how cookies should be serialized

[sbingler]

The spec should specify how document.cookie serializes cookie octets into a cookie-string (To my untrained eye it looks like the existing document.cookie section does something like this, but I'm not sure if it's sufficient)

While this was previously handled (somewhat hand wavy) by a note within rfc6265bis, it was recently decided that the HTML spec might be a better choice for these instructions. https://github.com/httpwg/http-extensions/issues/1502