hdf5doc

Purpose:
    To more carefully describe the
        -- behavior of H5Pset_external
        -- appropriate usage of H5Pset_shuffle

Description:
    H5Pset_external
      Add notes that first H5Pset_external call sets the dataset as EXTERNAL
      and identifies the first file in the series of external files that
      will hold the dataset; subsequent calls identify additional files;
      all external files must be declared before the dataset is created;
      and the library will create files that don't yet exist on the system
      at the time that H5Dwrite is called to write data to that file.
    H5Pset_shuffle
      Added notes regarding usage of H5Pset_shuffle in concert with a
      compression filter.
    And, as always, a few copy edits.