Document basic set functions in Partitions.thy for general non-partition arguments

[clange]

In Partitions.thy, the documentations of definition coarser_partitions_with and definition coarser_partitions_with_list currently assume that the argument P is a partition. In practice P will be a partition but we can't be sure of that. We should also prove that, when P is a partition, coarser_partitions_with behaves as expected. (OK, actually it does, as certified by the overall equivalence proof for all_partitions*.)