bug6716: documenting reserves and old_reserves tables

bug6716: documenting reserves and old_reserves tables

This patch documents the reserves and the old_reserves table.
Note that a few fields are marked unused: notificationdate and
reminderdate. Also two fields are left undocumented: lowestPriority
which never seems to be anything other than 0 and constrainttype which
is either 'a' or 0, but there doesn't appear to be any explanation
as to what each stands for.

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>