Use links than plain text references.

TODO: Internationalize the RDoc hardcoded link text.

Currently RDoc hardcodes reference name with external name in English,
as "REF at <code>FILE</code>".  To this currently, square brackets
with additional text is mandatory.

In rdoc files, all documents belong to the top level of each file, and
`:enddoc:` turns off `#document_self` for the entire document in the
current file.  This disables cross-references to labels in that file.
Instead use `:stopdoc:` so that parsing can re-start by `:startdoc:`
after the local variables.

Refine the uses of word "Data", which were often ambiguous.  Also,
that word now refers the new class unrelated to `T_DATA`.