Help text injected by CustomHelpFormatter is not included in rendered docs

[victorlin]

from #1705 (review)

#1705 appends text to the end of certain arguments in the --help output. However, these are not shown in the rendered docs.

Example for augur curate format-dates:

  --metadata-delimiters METADATA_DELIMITERS [METADATA_DELIMITERS ...]
                        Delimiters to accept when reading a plain text metadata file. Only
                        one delimiter will be inferred. Specified values will override the
                        default list. (default: (',', '\t'))