[css-toggles] Clarify what "persistent state" means

[miketaylr]

In https://tabatkins.github.io/css-toggle/#toggles (and elsewhere), the notion of "persistent state" is used:

Toggles are persistent state on an element, and are not directly affected by any CSS properties.

It may be useful to define this, or clarify in a security and privacy considerations section that toggles aren't cookies, or storage. Or to define what persistent state means so it's more obvious.

[tabatkins]

Indeed, this is meant to be page-lifetime state, same as the contents of a form element. It can persist in the BF cache but no further; anything more requires script to actually persist it somehow. The lifetime considerations aren't defined at all right now, which needs fixing.