argp options marked as OPTION_DOC are displayed (with the leading --, even)
on --usage output.  This does not make sense, these are not to be treated as
options by the parser, so they certainly should not be displayed by --usage,
let alone with the unexistent leading double-dashes.

This problem can be worked around by always using OPTION_DOC |
OPTION_NO_USAGE, but the correct behaviour would be for OPTION_DOC to imply
also OPTION_NO_USAGE.

tags 296367 + patch
quit

It would be more flexible to just have the usage messages display
`OPTION_DOC' entries the same way that help messages do (e.g.,
without the `--' prefix).  `OPTION_NO_USAGE' still works to
prevent these options from displaying in the usage.

`OPTION_DOC' is useful to specify the format of complex options
that are not displayed properly by default.  Having them appear
in the usage message in these cases keeps things consistent.