[css-syntax-3] Fix description of identifier (w3c#6122)

Author: Masa-Shin

css-syntax-3/Overview.bs

@@ -137,8 +137,9 @@ Description of CSS's Syntax</h2>
 	</div>
 
 	Property names and <a>at-rule</a> names are always <a>identifiers</a>,
-	which have to start with a letter or a hyphen followed by a letter,
-	and then can contain letters, numbers, hyphens, or underscores.
+	which have to start with an <a>identifier-start code point</a>, two hyphens,
+	or a hyphen followed by an identifier-start code point,
+	and then can contain zero or more <a>identifier code points</a>.
 	You can include any <a>code point</a> at all,
 	even ones that CSS uses in its syntax,
 	by <a>escaping</a> it.