hdf5doc

Purpose:
    Fix Bugzilla # 196 in UG.  (Previously fixed in RM.)
Description:
    Added paragraph to UG, "Datasets" chapter, section 4.5, "Chunked" subsection, 
        specifying that on chunk size cannot be larger than the dimensions of a 
        fixed-size dataset.  Moved rank limitation to same paragraph. (1.6 and 1.7)
    Other minor copy edits. (1.8)
    Closes Bugzilla entry 196.