Document the structure of the NeXus link in HDF5

[prjemian]

In fileformat.rst, there are descriptions of the basic NeXus data components.

Documentation of the link is marked as TODO: and points to an example from h5dump.

Look for this text in the source file: TODO: write some text about HDF5 hard links

Document both HDF5 and NeXus links (especially important since those reading this part will need to know to use an HDF5 HARDLINK and add the @target attribute).

[prjemian]

This issue is related to #388

[prjemian]

Also, change the text innxdl.xsd with respect to the example of a target attribute in the linkType definition. The present example is misleading in that it suggest one should use a NeXus class path as the value of the target attribute. That is not correct. The value should be the absolute HDF5 address of an existing group or field in the data file.

[prjemian]

The example use of the target attribute in the Design chapter is sufficient documentation.

Open a new issue if there is a suggestion about how to better describe the structure of a link or advertise the location of this documentation.