Update coolwsd.xml.in add concrete link to frame-ancestor

cc @hfiguiere

coolwsd.xml.in

@@ -184,7 +184,7 @@
         <host desc="Ditto, but as IPv4-mapped IPv6 addresses">::ffff:10\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}</host>
         <host desc="Localhost access by name">localhost</host>
       </lok_allow>
-      <content_security_policy desc="Customize the CSP header by specifying one or more policy-directive, separated by semicolons. See w3.org/TR/CSP2"></content_security_policy>
+      <content_security_policy desc="Customize the CSP header by specifying one or more policy-directive, separated by semicolons. See https://www.w3.org/TR/CSP2/#directive-frame-ancestors"></content_security_policy>
       <frame_ancestors desc="OBSOLETE: Use content_security_policy. Specify who is allowed to embed the Collabora Online iframe (coolwsd and WOPI host are always allowed). Separate multiple hosts by space."></frame_ancestors>
       <connection_timeout_secs desc="Specifies the connection, send, recv timeout in seconds for connections initiated by coolwsd (such as WOPI connections)." type="int" default="30"></connection_timeout_secs>