Adds more info to 404.

Meanwhile, the change to p2obp has tested OK, so the previous commit
closes #2562

Author: JJ

doc/404.pod6

@@ -6,9 +6,11 @@
 
 <p style="line-height: 1.7em">
     The page you tried to reach cannot be found. If you got here by following
-    a link on this site, please report it
+    a link on this site or as a result of searching on it, please report it
     <a href="https://github.com/perl6/doc/issues/">on our GitHub repository</a>
     so we can fix it!
+    If a search engine sent you here, the link might have disappeared due to site reorganization. We still encourage you to report it as above, but it will eventually disappear from search engines too.
+    If you came following a link from some site or blog, we would still want to know about that; you might want to report the blog or website author too. 
 </p>
 
 =end Html