[css-media-queries-5] Add security and privacy section to "prefers-reduced-data"

[Spec Link](https://drafts.csswg.org/mediaqueries-5/#prefers-reduced-data)

During [the WG call](https://github.com/w3c/csswg-drafts/issues/2370#issuecomment-594960678) it was requested/proposed that a section regarding security and privacy be added to the spec for the media query `prefers-reduced-data` to cover fingerprinting concerns. 