Notify users about the "fix track lengths" script track-time-setting process when they attach a TOC

From http://bugs.musicbrainz.org/ticket/4638

Currently it's not clear to users when they attach a TOC (to a release with no track times, especially) that the system will set the track times automagically in a batch process overnight.

A simple note added to the success message with a pointer to the documentation would probably achieve this.

This will be partially mitigated by MBS-1662 for tracks without times.