hdf5doc

I copied the trunk version of the file format spec to the 1.8 and 1.8.16 
branches. The last change made to both the trunk and 1.8 versions was 
revision 5543 on 11.7.2014. Since then Quincey made one change in June 2015. 
That change is in revision 5718. The trunk message is the following: 
"Description: Document new 'fail if unknown always' object header 
message flag." I made other changes from late September 2015 through 
early November 2015. Those changes were minor fixes, many of which were 
formatting changes. In the last two revisions (made in November 2015), 
I updated links to the new FM generated UG.