[dox] Fix screwed up list nesting in getopt's ddoc comment.

`getopt`'s ddoc was badly screwed up due to malformed nested lists, causing dmd to get confused and complain about mismatching parentheses (even though they actually do match -- albeit in a screwed up way). This PR cleans up (at least the initial part of) the mess, to use a uniform list for describing various option types.
I didn't go through the entire ddoc comment yet, but this is fixed the ddoc warning so I'm leaving it at that.