second. The first parameter can be either a page name, or some other
destination file, as registered by `will_render`.
+Provide a second parameter whenever possible, since this leads to better
+behaviour for the [[plugins/po]] plugin and `file:///` URLs.
+
If the second parameter is not specified (or `undef`), the URL will be
valid from any page on the wiki, or from the CGI; if possible it'll
be a path starting with `/`, but an absolute URL will be used if