httpauth: Recommend if_not_empty parameter for REMOTE_USER

This is untested, but should hopefully avoid the failure mode
described in [[bugs/Anon_edit_caused_lock_out_on_entire_site_]].

doc/plugins/httpauth.mdwn

@@ -41,6 +41,6 @@ users authentication via other methods.
 You have to pass the $remote_user variable to the CGI:
 
     location /ikiwiki.cgi {
-        fastcgi_param REMOTE_USER $remote_user;
+        fastcgi_param REMOTE_USER $remote_user if_not_empty;
         ....
     }