ls-files: add missing documentation for --resolve-undo option

ls-files: add missing documentation for --resolve-undo option

ls-files' --resolve-undo option has existed ever since 9d9a2f4aba
("resolve-undo: basic tests", 2009-12-25), but was never documented.
However, the option has been referred to in the ls-files manual itself
ever since ce74de931d ("ls-files: introduce "--format" option",
2022-07-23), making its omission a bit jarring.  Document this option.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>