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>

Documentation/git-ls-files.txt

@@ -118,6 +118,7 @@ OPTIONS
 	linkgit:git-status[1] `--short` or linkgit:git-diff[1]
 	`--name-status` for more user-friendly alternatives.
 +
+--
 This option identifies the file status with the following tags (followed by
 a space) at the start of each line:
 
@@ -128,6 +129,7 @@ a space) at the start of each line:
 	C::	modified/changed
 	K::	to be killed
 	?::	other
+--
 
 -v::
 	Similar to `-t`, but use lowercase letters for files