improve gpx bbox field description

www/templates/maposmatic/new.html

@@ -212,7 +212,7 @@ will be visible on the map.{% endblocktrans %}
                   {{ form.track }}
                 </fieldset>
                 <fieldset id="track-bbox-mode">
-                  <legend>{% trans "BBOX mode" %}</legend>
+                  <legend>{% trans "What to do with the current selction bounding box? Keep as is, extend to fully include the GPX track ('merge'), or completely replace with the tracks bounding box?" %}</legend>
                   {{ form.track_bbox_mode }}
                 </fieldset>
               </div>