[css-cascade-4] Inconsistency between "value space" and "value syntax"

[gsnedders]

css-backgrounds-3/Overview.bs
2675:      since other properties will be eliding the three-value syntax.

css-cascade-3/Overview.bs
50:	a value space

css-cascade-4/Overview.bs
81:	a value space
425:			When the old property’s value syntax is identical
1353:			Removed possibility of [=legacy name aliases=] to map subsets of the value space,

css-cascade-5/Overview.bs
55:	a value space
383:			When the old property’s value syntax is identical

css-flexbox-1/Overview.bs
3733:			Updated value syntax in property definition tables

css-font-loading-3/Overview.bs
756:			using the CSS value syntax of the 'font' property.

css-text-4/Overview.bs
6083:		Suggestion for value space is ''match-indent | <<length>> | <<percentage>>''

css-values-3/Overview.bs
173:	For readability these are not listed explicitly in the property value syntax definitions.
1660:	Note: The 'background-position' property also accepts a three-value syntax.
2507:		<li>Inlined the <<position>> definition and dropped the three-value syntaxes

css-values-4/Overview.bs
175:	For readability these are not listed explicitly in the property value syntax definitions.
2638:	Note: The 'background-position' property also accepts a three-value syntax.

css-variables-1/Overview.bs
374:Custom Property Value Syntax</h3>

css-variables-2/Overview.bs
364:Custom Property Value Syntax</h3>

cssom-1/Overview.bs
2734:part of the property value space and will therefore cause
2764:        (a pattern typically represented by a double bar <a grammar>||</a> in the value syntax),

Notably, we mostly use "Value Syntax" and not "Value Space" aside from one use in Cascade (in the intro), one use in CSSOM (in a Note), and one use in Text (in an Issue).

I guess arguably we could make a distinction between "Value Syntax" (i.e., the formal grammar) and the "Value Space" (i.e., the formal language), which do (of course) have a correspondence. But it seems silly to use "value space" a few times informatively when we use "Syntax" everywhere normatively.

[tabatkins]

Yeah, no intended difference between those. Fix PRs happily accepted. ^_^