[CSS] add `length snapped as a line width` reference

[sb3nder]

MDN URL

https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Values/length

What specific section or headline is this issue about?

No response

What information was incorrect, unhelpful, or incomplete?

In this PR mdn/data#1069,
I'm updating the computed value for border-*-width, column-rule-width, outline-width, outline-offset, -webkit-border-*-width, to "the absolute length, snapped as a line width" .
There is no docs explaing what that means, that I can easely link in the computed value description.

There are also a few old nits to fix regarding the computed values being untangled from the *-style properties’ values.

What did you expect to see?

A more or less detailed explanation of what "snapped as a line width" means.

Do you have any supporting links, references, or citations?

border-*-width, column-rule-width, outline-width snapped as a line width:
w3c/csswg-drafts#5210

Computed value untangled from *-style properties value:
w3c/csswg-drafts#11494 (comment)

outline-offset snapped as a line width:
w3c/csswg-drafts#12906

Definition of "length snapped as a line width":
https://drafts.csswg.org/css-values-4/#snap-as-a-line-width

Do you have anything more you want to share?

No response