(#10321) Fix array support in schedule's range parameter

Change the schedule type's range parameter to properly evaluate
all elements of a supplied array for validity instead of only
checking the first member of the array. Add documentation to
clarify that range does accept an array.

Fix the associated tests to use must instead of should (Puppet::Type#should
shadows the rspec should).

Signed-off-by: Patrick Carlisle <patrick@puppetlabs.com>