Clarify contracts of toArray methods

As requested in https://github.com/wmde/WikibaseDataModel/pull/311#issuecomment-64713212.

Please note that `StatementList` always reindexed the array. It stopped doing this with #285 (released with 2.4.0).

I think this must be the responsibility of the `StatementList` class (as well as `ClaimList`, see the TODO) and not the calling code.