Add better language and link to Upgrading page in docs for relative permalinks deprecation msg

lib/jekyll/site.rb

@@ -424,9 +424,10 @@ module Jekyll
       if config['relative_permalinks'] && !@deprecated_relative_permalinks
         $stderr.puts # Places newline after "Generating..."
         Jekyll::Logger.warn "Deprecation:", "Starting in 1.1, permalinks for pages" +
-                                            " in subfolders must be absolute" +
-                                            " permalinks relative to the site" +
-                                            " source."
+                                            " in subfolders must be relative to the" +
+                                            " site source directory, not the parent" +
+                                            " directory. Check http://jekyllrb.com/docs/upgrading/"+
+                                            " for more info."
         $stderr.print Jekyll::Logger.formatted_topic("") + "..." # for "done."
         @deprecated_relative_permalinks = true
       end