Use of .document with RDoc

Suppose I've installed the rdoc (2.5.8) gem without creating any
documentation. It has a top level file, .document, that contains:

	History.txt
	README.txt
	RI.txt
	lib

I move to the directory .../gems/rdoc-2.5.8 and run the command:

	rdoc --ri --op /tmp/doc

I expect to get all relevant ri documentation in /tmp/doc but I see
nothing corresponding to the .txt files mentioned in .document.

I would be grateful if someone could explain to me what I am doing
wrong and, even more importantly, what I should do to get the
documentation corresponding to the .txt files.

Barrie.