hdf5doc

To fix Bugzilla 1205, I added the following sentence to 
the end of section 4.1 “Data Selection” of the Dataspaces 
chapter: 

The definition of a selection within a dataspace, not 
the data in the selection, cannot be saved to the file 
unless the selection definition is saved as a region 
reference. See the “References to Dataset Regions” 
section for more information.

Tested in Firefox.