PlanarImg: make javadoc more correct

The previous text was a copy-paste from ImagePlusImg (formerly
ImagePlusContainer) and had much leftover text from that transition.
ImagePlusImg: make javadoc more correct

Some questionable statements remain, particularly the bit about being
able to create an ImagePlusImg around an existing ImagePlus -- is this
really possible using the ImagePlusImg class itself? Or must one of
the subclasses be used?
ImagePlusImg: remove bogus parameter declaration