Issue 434 - Added documentation for $sitekey functionality

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 <p>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><em>Disclaimer</em>: All filter examples given here are really only examples and are not meant to be used.</p>
 
   <h2 id="introduction">Introduction to Adblock Plus filters</h2>
 
   <p>The options described in this section should be enough for users who have to create a filter occasionally.</p>
 
   <h3 id="basic">Basic filter rules</h3>
 
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         <li><code><fix>other</fix></code> -- types of requests not covered in the list above</li>
       </ul>
       The type options <code><fix>background</fix></code>, <code><fix>xbl</fix></code>,
       <code><fix>ping</fix></code> and <code><fix>dtd</fix></code> are outdated and should no
       longer be used.
     </li>
     <li>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>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>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>
-      Sitekey restrictions: The option <code><fix>sitekey=abcdsitekeydcba</fix></code> means that the filter should only be applied on pages that provide a public key and a signature which can be verified by that very same public key that is also contained in the filter (but without the trailing =). Multiple sitekeys can be specified using "|" as separator: with the option <code><fix>sitekey=abcdsitekeydcba|bcdesitekeyedcb</fix></code> the filter will only be applied on pages providing either sitekey "abcdsitekeydcba" or "bcdesitekeyedcb". Note that sitekey restrictions require <a href="#sitekey_server">modifications on the server-side</a>.

[Wladimir Palant, 2014/09/08 14:09:26]

Maybe add something like "This is similar to domain restrictions but allows
covering scenarios where a single filter should apply to a very large number of
domains."

[Thomas Greiner, 2014/09/08 15:17:36]

Done.

+      Sitekey restrictions: The option <code><fix>sitekey=abcdsitekeydcba</fix></code> means that the filter should only be applied on pages that provide a public key and a signature which can be verified by that very same public key that is also contained in the filter (but without the trailing =). Multiple sitekeys can be specified using "|" as separator: with the option <code><fix>sitekey=abcdsitekeydcba|bcdesitekeyedcb</fix></code> the filter will only be applied on pages providing either sitekey "abcdsitekeydcba" or "bcdesitekeyedcb". This is similar to domain restrictions but allows covering scenarios where a single filter should apply to a very large number of domains. Note that sitekey restrictions require <a href="#sitekey_server">modifications on the server-side</a>.
     </li>
     <li><code><fix>match-case</fix></code> -- makes the filter only apply to addresses with matching letter case, e.g. the filter <code><fix>*/BannerAd.gif$match-case</fix></code> will block <code><fix>http://example.com/BannerAd.gif</fix></code> but not <code><fix>http://example.com/bannerad.gif</fix></code>.</li>
     <li><code><fix>collapse</fix></code> -- this option will override the global "Hide placeholders of blocked elements" option and make sure the filter always hides the element. Similarly the <code><fix>~collapse</fix></code> option will make sure the filter never hides the element.</li>
     <li>
       <code><fix>donottrack</fix></code> -- for any address matching a blocking rule with
       this option and not matching any exception rules with this option a
       <a href="http://donottrack.us/">Do-Not-Track header</a> will be sent (requires
       Adblock Plus 1.3.5 or higher).<fix> </fix>For backwards compatibility it is recommended
       to use this option in combination with contradicting type options, this will prevent this
       filter from blocking anything in earlier Adblock Plus versions:
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   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>
   Finally, generate the signature for this string by using the signature algorithm SEC_OID_ISO_SHA_WITH_RSA_SIGNATURE (default when using OpenSSL).
 </p>