[css-borders-4][editorial] Rewrite syntaxes with property value range productions

[cdoublev]

It might not be obvious why property value range productions should be preferred (imo), so taking corner-* shorthands as an example, its current syntax is:

<length-percentage [0,∞]>{1,2} || <corner-shape-value>.

When replaced with <'border-top-left-radius'> || <'corner-top-left-shape'>:

• readers know immediately that the first value is a radius (and the second is a shape)
• spec authors will not need to update it if the syntax of corner-*-shape is later updated to eg. <corner-shape-value> | none
• implementers do not need to look up what the second <length-percentage> value should be when omitted
• implementers may have to write less code to validate a grammar rule applying on the longhand value, to expand the shorthand value, and serialize a value according to the shortest value principle

Applying this change left the extended definition of <corner-shape-value> at an inappropriate place, below the definition table of corner-shape, while it was no longer used in its syntax.

So I took changes a little further and ended with moving/rewriting corner shaping sections to make all *Corners" sections more consistent, in a separate commit.

I also made an unrelated editorial change. Although experienced CSS authors might intuitevily guess this, I suggest to explicitly defining that corner-shape values are expanded clockwise starting from the top corner, and what their default values when omitted. I preferred to suggest this change in a single PR to avoid merge conflicts.

[noamr]

Nice, thanks for this!