Provide better documentation of "declarationDate" and "contributionYear"

[montanajava]

The actual meaning of these attributes is unclear. It would be nice to have a few more words on each topic, also so as to delineate the difference between the two.

[montanajava]

After conversation with domain expert from BLW:

The following is not complete, but you can use it as a starting point for improvement.

declarationDate: [EN] Quantity of animals as of the declaration day (01 January of the survey year). [DE] Anzahl Tiere am Stichtag (01. Januar des Erhebungsjahres).

contributionYear: [EN] Year in which the respective registration of a programme is valid. [DE] Jahr, für welches die entsprechende Anmeldung eines Programms gilt.

surveyYear: [EN] Year in which data was collected. [DE] Jahr, in welchem die Daten erhoben wurden.