[css-variables] Improve note about custom properties. Add DoC.

--HG--
extra : rebase_source : 62591cae5197bc2d00a0370920d766fa63eef35e

Author: tabatkins

css-variables/Overview.bs

@@ -118,8 +118,10 @@ Defining Custom Properties: the '--*' family of properties</h2>
 	and thus are not substituted in by the corresponding variable.
 
 	Note: While this module focuses on the use of <a>custom properties</a> with the ''var()'' function to create “variables”,
-	it's expected that a future module will make it easier to use them as actual custom properties,
+	they can also be used as actual custom properties,
 	parsed by and acted on by script.
+	It's expected that the CSS Extensions spec [[CSS-EXTENSIONS]]
+	will expand on these use-cases and make them easier to do.
 
 <h3 id='syntax'>
 Custom Property Value Syntax</h3>

css-variables/Overview.html

@@ -229,8 +229,10 @@ <h2 class="heading settled heading" data-level=2 id=defining-variables><span cla
 	and thus are not substituted in by the corresponding variable.
 
 <p class=note>	Note: While this module focuses on the use of <a data-link-type=dfn href=#custom-property title="custom properties">custom properties</a> with the <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function to create “variables”,
-	it’s expected that a future module will make it easier to use them as actual custom properties,
+	they can also be used as actual custom properties,
 	parsed by and acted on by script.
+	It’s expected that the CSS Extensions spec <a data-biblio-type=informative data-link-type=biblio href=#css-extensions title=css-extensions>[CSS-EXTENSIONS]</a>
+	will expand on these use-cases and make them easier to do.
 
 <h3 class="heading settled heading" data-level=2.1 id=syntax><span class=secno>2.1 </span><span class=content>
 Custom Property Value Syntax</span><a class=self-link href=#syntax></a></h3>