[css-variables] Clarify variable interpolation bheavior.

Author: tabatkins

css-variables/Overview.bs

@@ -183,6 +183,10 @@ Custom Property Value Syntax</h3>
 	can be made conditional with ''@media'' and other conditional rules,
 	can be used in HTML's <code>style</code> attribute,
 	can be read or set using the CSSOM, etc.
+	Notably, they can even be transitioned or animated,
+	but since the UA has no way to interpret their contents,
+	they always use the "flips at 50%" behavior
+	that is used for any other pair of values that can't be intelligently interpolated.
 
 	<div class='example'>
 		This style rule:

css-variables/Overview.html

@@ -56,7 +56,7 @@
 </p>
   <h1 class="p-name no-ref" id=title>CSS Custom Properties for Cascading Variables Module Level 1</h1>
   <h2 class="no-num no-toc no-ref heading settled heading" id=subtitle><span class=content>Editor’s Draft,
-    <span class=dt-updated><span class=value-title title=20140401>1 April 2014</span></span></span></h2>
+    <span class=dt-updated><span class=value-title title=20140403>3 April 2014</span></span></span></h2>
   <div data-fill-with=spec-metadata><dl><dt>This version:<dd><a class=u-url href=http://dev.w3.org/csswg/css-variables/>http://dev.w3.org/csswg/css-variables/</a><dt>Latest version:<dd><a href=http://www.w3.org/TR/css-variables-1/>http://www.w3.org/TR/css-variables-1/</a><dt>Editor’s Draft:<dd><a href=http://dev.w3.org/csswg/css-variables/>http://dev.w3.org/csswg/css-variables/</a><dt>Previous Versions:<dd><a href=http://www.w3.org/TR/2013/WD-css-variables-1-20130620/ rel=previous>http://www.w3.org/TR/2013/WD-css-variables-1-20130620/</a><dd><a href=http://www.w3.org/TR/2013/WD-css-variables-20130312/ rel=previous>http://www.w3.org/TR/2013/WD-css-variables-20130312/</a><dd><a href=http://www.w3.org/TR/2012/WD-css-variables-20120410/ rel=previous>http://www.w3.org/TR/2012/WD-css-variables-20120410/</a>
     <dt>Feedback:</dt>
         <dd><a href="mailto:www-style@w3.org?subject=%5Bcss-variables%5D%20feedback">www-style@w3.org</a>
@@ -292,6 +292,10 @@ <h3 class="heading settled heading" data-level=2.1 id=syntax><span class=secno>2
 	can be made conditional with <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/css-conditional-3/#at-ruledef-media title=@media>@media</a> and other conditional rules,
 	can be used in HTML’s <code>style</code> attribute,
 	can be read or set using the CSSOM, etc.
+	Notably, they can even be transitioned or animated,
+	but since the UA has no way to interpret their contents,
+	they always use the "flips at 50%" behavior
+	that is used for any other pair of values that can’t be intelligently interpolated.
 
 	<div class=example>
 		This style rule: