Typesetting long options in rdoc

I don’t succeed in nicely typesetting long options in itemized lists in
rdoc.
The RDOC documentation (RDoc Documentation) tells me this
example:

=begin rdoc

–output name [, name]::
specify the name of one or more output files. If multiple
files are present, the first is used as the index.

–quiet::: do not output the names, sizes, byte counts,
index areas, or bit ratios of units as
they are processed.

=end

But the result (see http://www.servalys.nl/doc/ ) is that the – are
seen as a single long hyphen (actually —), while I want to see a
hyphen, a little white space and then another hyphen.

When I insert a space between the to hyphens, the result is two very
narrow hyphens with excessive whitespace in between.

Is there a better way to do this?