Add cref describing the situation with HTML forms

[handrews]

The specification is currently in a slightly awkward state of both
drawing analogies with HTML forms and supporting a different, more
explicit paradigm with "hrefSchema".

Add a note acknowledging the confusion and providing some context
for the current state of things.

[handrews]

Paging @jdesrosiers to see if this explanation will help resolve #280.

[jdesrosiers]

I have no objection. I agree that we are moving away from LDO having direct parallels to a HTML Forms.

But, I don't think the form analogy is the root problem with #280, even though I used it to express my thoughts at one point. My latest comment on that PR I think better explains the problem and does it without the form analogy.

[handrews]

@jdesrosiers agreed about this and #280 given our subsequent discussions. I still think this would be a useful note, as long as @awwright and @Relequestual agree, so I'll leave it open.

[handrews]

I'm going to close this because of #292 and #293, and the plan to add a third PR to address the "form" use case. Something like this should probably be included in the "form" PR, but it will look different from this so let's get this out of the way to reduce noise.