The output of `darcs help markdown` should be, in the following order:
1. commands reference
3. files that live in _darcs/prefs.
4. environment variables.
Currently it's 1, 4, 2. I believe it is okay to add 3 in the source code
of Darcs the same way 4 is present, this would avoid documentation rot?
4. is available on the wiki at http://darcs.net/Using/Configuration