[tldoc] doc changes redux

[Karl Berry]

Sadly, we had to make some more changes to the (English) TL
documentation, relating to the "portable" feature/section.  So please
update your translations if you have time.

Also, I made \OnCD a no-op, since the DVD doesn't have unpacked files
any more and the macro never really worked in other cases anyway.  Not
worth investing more time in.  So if you have your own copy of the
tex-live.sty definitions, feel free to change it accordingly.

I sincerely hope no more changes will be deemed necessary ...

karl