Add prose definition for escapes (already in grammar, but no mention in prose!)

Author: fantasai

selectors3/Overview.html

@@ -682,6 +682,10 @@ <h2><a name=selector-syntax>4. Selector syntax</a></h2>
 no pseudo-element, is an <a href="#Conformance">invalid
 selector</a>.</p>
 
+<p>Characters in Selectors can be escaped with a backslash according
+to the same <a href="http://www.w3.org/TR/CSS21/syndata.html#characters">escaping
+rules</a> as CSS. <a href="#refsCSS21">[CSS21]</a>.</p>
+
 <p id="nsdecl">Certain selectors support namespace prefixes. The
 mechanism by which namespace prefixes are <dfn>declared</dfn> should
 be specified by the language that uses Selectors. If the language does