Amend DATEDIFF documentation for unit millisecond

[mrotteveel]

In Firebird 3.0.8 and 4.0.1, by way of FirebirdSQL/firebird#6987, the return type of DATEDIFF for unit MILLISECOND was changed from BIGINT to NUMERIC(18, 1). Update fblangref30 and fblangref40 with this information.

[mrotteveel]

Published new versions of fblangref30, fblangref30-de, fblangref40, fblangref40-de