Currently we special-case "darcs help --match" to provide (inadequate)
documentation on the match EDSL (embedded domain-specific language).

We can/should also provide information on Darcs environment variables
(e.g. DARCS_EMAIL) and Darcs files (e.g. ~/.darcs/sources).  Currently
this information is only available in the user manual, but it can and
should be available from the command line and in the manpage, too.

As a mock-up, consider how Mercurial does it:

    $ hg --help
    list of commands:
     <A COMMAND>  <ITS SYNOPSIS>
     [...]        [...]

    additional help topics:
     dates        Date Formats
     patterns     File Name Patterns
     environment  Environment Variables
     revisions    Specifying Single Revisions
     multirevs    Specifying Multiple Revisions
     diffs        Diff Formats
     templating   Template Usage
     urls         Url Paths

    $ hg help environment
    Environment Variables

    EDITOR::
        Sometimes Mercurial needs to open a text file in an editor
        for a user to modify, for example when writing commit messages.
        The editor it uses is determined by looking at the environment
        variables HGEDITOR, VISUAL and EDITOR, in that order. The first
        non-empty one is chosen. If all of them are empty, the editor
        defaults to 'sensible-editor'.

    [...]

The following patch updated the status of issue1455 to be resolved:

* Resolve issue1455: implement "darcs help environment". 
Ignore-this: 4db59cb774cfc06af64c0d7095b8923b