Issue 4669 - Mention filter option on filters documentation page

pages/filters.html

 title=Writing Adblock Plus filters
 
 
 <p>{{s1 Current Adblock Plus versions allow you to "tweak" your filters in many different ways. This document explains the choices that you have and how they can be used.}}</p>
 
   <p>{{s2 <em>Disclaimer</em>: All filter examples given here are really only examples and are not meant to be used.}}</p>
 
   <h2 id="introduction">{{s3 Introduction to Adblock Plus filters}}</h2>
 
   <p>{{s4 The options described in this section should be enough for users who have to create a filter occasionally.}}</p>
@@ skipping 139 matching lines @@
         <li>{{xmlhttprequest <code><fix>xmlhttprequest</fix></code> — requests started using the <a href="https://xhr.spec.whatwg.org/"><code><fix>XMLHttpRequest</fix></code> object</a> or <a href="https://fetch.spec.whatwg.org/"><code><fix>fetch()</fix></code> API</a>}}</li>
         <li>{{s63 <code><fix>object-subrequest</fix></code> — requests started by plugins like Flash}}</li>
         <li>{{s64 <code><fix>subdocument</fix></code> — embedded pages, usually included via HTML frames}}</li>
         <li>{{ping <code><fix>ping</fix></code> — requests started by <a href="https://developer.mozilla.org/docs/Web/HTML/Element/a#attr-ping"><code><fix>&lt;a ping&gt;</fix></code></a> or <a href="https://developer.mozilla.org/docs/Web/API/Navigator/sendBeacon"><code><fix>navigator.sendBeacon()</fix></code></a> (Adblock Plus 2.7.1 or higher required)}}</li>
         <li>{{websocket <code><fix>websocket</fix></code> — requests initiated via <a href="https://developer.mozilla.org/docs/Web/API/WebSocket"><code><fix>WebSocket</fix></code> object</a> (Adblock Plus 2.8 or higher required)}}</li>
         <li>{{webrtc <code><fix>webrtc</fix></code> — connections opened via <a href="https://developer.mozilla.org/docs/Web/API/RTCPeerConnection"><code><fix>RTCPeerConnection</fix></code> instances</a> to ICE servers (Adblock Plus for Chrome and Opera 1.13.3 or higher required, Adblock Plus for Firefox and Safari not yet supported)}}</li>
         <li>{{s65 <code><fix>document</fix></code> — the page itself (only <a href="#whitelist">exception rules</a> can be applied to the page)}}</li>
         <li>{{s66 <code><fix>elemhide</fix></code> — for exception rules only, similar to <code><fix>document</fix></code> but only disables <a href="#elemhide">element hiding rules</a> on the page rather than all filter rules (Adblock Plus 1.2 or higher required)}}</li>
         <li>{{generichide <code><fix>generichide</fix></code> — for exception rules only, similar to <code><fix>elemhide</fix></code> but only disables <a href="#generic-specific">generic</a> element hiding rules on the page (Adblock Plus 2.6.12 or higher required)}}</li>
         <li>{{genericblock <code><fix>genericblock</fix></code> — for exception rules only, just like <code><fix>generichide</fix></code> but disables <a href="#generic-specific">generic</a> blocking rules (Adblock Plus 2.6.12 or higher required)}}</li>
+        <li>{{popup <code><fix>$popup</fix></code> — pages opened in a new tab or window}}</li>
         <li>{{s67 <code><fix>other</fix></code> — types of requests not covered in the list above}}</li>
       </ul>
       {{deprecated-options The type options <code><fix>background</fix></code>,
       <code><fix>xbl</fix></code> and <code><fix>dtd</fix></code> are outdated
       and should no longer be used.}}
     </li>
     <li>{{s69 Inverse type options: specify the element types the filter should <strong>not</strong> be applied to. Possible inverse type options: <code><fix>~script</fix></code>, <code><fix>~image</fix></code>, <code><fix>~stylesheet</fix></code>, <code><fix>~object</fix></code>, <code><fix>~xmlhttprequest</fix></code>, <code><fix>~object-subrequest</fix></code>, <code><fix>~subdocument</fix></code>, <code><fix>~document</fix></code>, <code><fix>~elemhide</fix></code>, <code><fix>~other</fix></code>}}</li>
     <li>{{s70 Restriction to third-party/first-party requests: If the <code><fix>third-party</fix></code> option is specified, the filter is only applied to requests from a different origin than the currently viewed page. Similarly, <code><fix>~third-party</fix></code> restricts the filter to requests from the same origin as the currently viewed page.}}</li>
     <li>{{s71 Domain restrictions: The option <code><fix>domain=example.com</fix></code> means that the filter should only be applied on pages from "example.com" domain. Multiple domains can be specified using "|" as separator: with the option <code><fix>domain=example.com|example.net</fix></code> the filter will only be applied on pages from "example.com" or "example.net" domains. If a domain name is preceded with "~", the filter should <strong>not</strong> be applied on pages from this domain. For example, <code><fix>domain=~example.com</fix></code> means that the filter should be applied on pages from any domain but "example.com" and <code><fix>domain=example.com|~foo.example.com</fix></code> restricts the filter to the "example.com" domain with the exception of "foo.example.com" subdomain.}}</li>
     <li>
@@ skipping 101 matching lines @@
   {{s110 The data used for creating the signature is a concatenated list of request variables (namely URI, host and user agent) separated by the <code><fix>NUL</fix></code> character "\0". For example:}}
 </p>
 
 <pre>
   /index.html?q=foo\0www.example.com\0Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:30.0) Gecko/20100101 Firefox/30.0
 </pre>
 
 <p>
   {{s111 Finally, generate the signature for this string by using the signature algorithm SEC_OID_ISO_SHA_WITH_RSA_SIGNATURE (default when using OpenSSL).}}
 </p>