Documentation: wrap blocks with "--"

Documentation: wrap blocks with "--"

The documentation for each of these options contains a list. After the
list, AsciiDoc interprets the continuation as a continuation of the
*list*, not as a continution of the larger block. As a result, we get
too much indentation. Wrap the entire blocks in "--" to fix this. With
Asciidoctor, this commit is a no-op, and the two programs now render
these identically.

These two files share the same problem and indeed, they both document
`--untracked-files` in quite similar ways. I haven't checked to what
extent that is intentional or warranted, and to what extent they have
simply drifted apart. I consider such an investigation and possible
cleanup as out of scope for this commit and this patch series.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>