Fixes #1725.

So far there are three commits to this PR:

1. Adding a dummy dfn for #concept-document-origin to link to later
2. Linking to it from every instance of "origin" in the HTML Standard that seemed to specifically reference a Document's origin (it is possible I missed a couple, but pretty sure I got most or all of them)
3. Finish up the paragraph for the dfn of #concept-document-origin, explaining when an origin is changed, and how this origin can differ from the origin of a Document's URL
   • Also making a few more instances of "origin" point to the new dfn specifically

I know the origin is changed at least in document.domain, which I mention in my dfn paragraph. It's possible there are more (and I'd like to know about them), but I don't think we need to make the "for example ..." exhaustive.

As a bonus, I think I got all of the wrapping right (but would like someone to take a look)... just in time for Christmas eh? (ba dum tsss)

/browsers.html ( diff )
/browsing-the-web.html ( diff )
/comms.html ( diff )
/dom.html ( diff )
/dynamic-markup-insertion.html ( diff )
/history.html ( diff )
/iframe-embed-object.html ( diff )
/images.html ( diff )
/interaction.html ( diff )
/interactive-elements.html ( diff )
/links.html ( diff )
/origin.html ( diff )
/parsing.html ( diff )
/web-messaging.html ( diff )
/webstorage.html ( diff )
/window-object.html ( diff )