doc/ls-files: put nested list for "-t" option into block

The description for the "-t" option contains a sub-list of all of the possible file status outputs. But because of the newline separating that list from the description paragraph, asciidoc treats the sub-list entries as a continuation of the overall options list, rather than as children of the "-t" description. We could fix it by adding a "+" before the sub-list to connect it to the rest of the "-t" text. But using a pair of "--" to delimit the block is perhaps more readable, and may have better compatibility with asciidoctor, as in 39a869b2f2 (Documentation/rev-list-options: wrap --date=<format> block with "--", 2019-03-30). The extra blank line comes from 5bc0e247c4 (Document ls-files -t as semi-obsolete., 2010-07-28), but the problem actually seems older than that. Before then, we did: -t:: some text... H:: cached M:: unmerged etc... but asciidoc also treats that as one big list. So this problem seems to have been around forever. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>