Encapsulate all the strangeness to do with finding out where to find RDoc files
We basically deal with three directories:
The 'system' documentation directory, which holds the documentation distributed with Ruby, and which is managed by the Ruby install process
The 'site' directory, which contains site-wide documentation added locally.
The 'user' documentation directory, stored under the user's own home directory.
There's contention about all this, but for now:
$datadir/ri/<ver>/system/…
$datadir/ri/<ver>/site/…
~/.rdoc