Issue 4539 - Document new $websocket filter type

pages/filters.html

Index: pages/filters.html
===================================================================
--- a/pages/filters.html
+++ b/pages/filters.html
@@ -146,16 +146,17 @@ title=Writing Adblock Plus filters
         <li>{{s57 <code><fix>script</fix></code> — external scripts loaded via HTML script tag}}</li>
         <li>{{s58 <code><fix>image</fix></code> — regular images, typically loaded via HTML img tag}}</li>
         <li>{{s59 <code><fix>stylesheet</fix></code> — external CSS stylesheet files}}</li>
         <li>{{s60 <code><fix>object</fix></code> — content handled by browser plugins, e.g. Flash or Java}}</li>
         <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>{{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>{{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