hidden/visible-selector: add note about performance

Fixes jquerygh-679
Closes jquerygh-682

Author: arthurvr

entries/hidden-selector.xml

@@ -20,6 +20,7 @@
     <p>How <code>:hidden</code> is determined was changed in jQuery 1.3.2. An element is assumed to be hidden if it or any of its parents consumes no space in the document. CSS visibility isn't taken into account (therefore <code>$( elem ).css( "visibility", "hidden" ).is( ":hidden" ) == false</code>). The <a href="http://blog.jquery.com/2009/02/20/jquery-1-3-2-released/">release notes</a> outline the changes in more detail.</p>
   </longdesc>
   <note id="jquery-selector-extension" type="additional" data-selector=":hidden"/>
+  <note id="hidden-forces-layout" type="additional"/>
   <example>
     <desc>Shows all hidden divs and counts hidden inputs.</desc>
     <code><![CDATA[

entries/visible-selector.xml

@@ -15,6 +15,7 @@
     <p>How <code>:visible</code> is calculated was changed in jQuery 1.3.2. The <a href="http://blog.jquery.com/2009/02/20/jquery-1-3-2-released/">release notes</a> outline the changes in more detail.</p>
   </longdesc>
   <note id="jquery-selector-extension" type="additional" data-selector=":visible"/>
+  <note id="hidden-forces-layout" type="additional"/>
   <example>
     <desc>Make all visible divs turn yellow on click.</desc>
     <code><![CDATA[

notes.xsl

@@ -58,6 +58,9 @@
 		<xsl:when test="@id = 'html-code-execution'">
 			By design, any jQuery constructor or method that accepts an HTML string — <a href="/jQuery/">jQuery()</a>, <a href="/append/">.append()</a>, <a href="/after/">.after()</a>, etc. — can potentially execute code. This can occur by injection of script tags or use of HTML attributes that execute code (for example, <code>&lt;img onload=""&gt;</code>). Do not use these methods to insert strings obtained from untrusted sources such as URL query parameters, cookies, or form inputs. Doing so can introduce cross-site-scripting (XSS) vulnerabilities. Remove or escape any user input before adding content to the document.
 		</xsl:when>
+		<xsl:when test="@id = 'hidden-forces-layout'">
+			Using this selector heavily can have performance implications, as it may force the browser to re-render the page before it can determine visibility. Tracking the visibility of elements via other methods, using a class for example, can provide better performance.
+		</xsl:when>
 	</xsl:choose>
 </xsl:template>
 </xsl:stylesheet>