[release,documentation] Fix documentation of PartitionIteratorType.

Surprisingly, the iterators over the partition type Overlap_Partition
and OverlapFront_Partition also visit iterior and border entities.
According to our partition check (dune/grid/test/checkpartition.cc) this
is even mandatory. Unfortunately this neither reflected in our papers
nor in the doxygen documentation. This patch at least documents it the
doxygen documentation and hopefully will save future grid implementors
hours of debugging time.