For third-parth plugins, in can be helpful if the 'More help' links in
help pop-ups (the ones that come from $string['..._link'] string in the
language file) can go to other places.
This change support two other sorts of URL in addition to the standard
'course/editing' type of link that goes to MoodelDocs.
You can use absolute URLs, starting http:// or https:///
You can use a link starting %%WWWROOT%%, and that token is replaced by
$CFG->wwwroot to make the link.
Tim Hunt