Bug 7136: Correct description of Grace period for subscriptions

Bug 7136: Correct description of Grace period for subscriptions

The grace period is used with the serialsUpdate.pl cronjob. It will
generate the next issue and set the one before to 'Late' with
an optional note.

There is on trace of a 'waiting' status for serials.

To test:
- check help file on the subscription edit form for
  the description of the 'Grace period'.

Signed-off-by: Maryse Simard <maryse.simard@inlibro.com>

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>