[css-values-3] Editorial sync with L4

* Restore accidentally dropped phrase about <number> syntax.
* Clarify that <resolution> can never be negative.
* Editorially rearrange <position> grammar.
* Minor wording adjustments.
* Add clarifying notes.
* Improve cross-linking.
* Fix/update markup and punctuation.
* Rewrap / fix indentation (e.g. valdef section).
* Add/update "Big Text" comments.

Author: fantasai