privacy concerns section should mention storage mechanisms

[dbaron]

While looking at w3ctag/design-reviews#411 I read the Privacy Concerns section of the HTML specification's introduction, and noticed that it didn't mention anything about storage mechanisms, a number of which are defined in the HTML specification, and which are a major privacy concern related to the HTML spec that seems like it should be mentioned in any section describing the privacy issues of the HTML spec.

@hober pointed out that @othermaciej expressed similar concerns in a review of a linked issue 5 hours earlier.

[annevk]

The "Web Storage" section has its own Privacy section and also its own Security section: https://html.spec.whatwg.org/#privacy. This organization follows from when those sections were published separately. It might make sense to reconcile those things a bit more or move various (largely considered legacy) storage mechanisms into https://storage.spec.whatwg.org/ to reduce the size of the HTML Standard.