ls-files: clarify descriptions of status tags for -t

ls-files: clarify descriptions of status tags for -t

Much like the file selection options we tweaked in the last commit, the
status tags printed with -t had descriptions that were easy to
misunderstand, and for many of the same reasons.  Clarify them.

Also, while at it, remove the "semi-deprecated" comment for "git
ls-files -t".  The -t option was marked as semi-deprecated in 5bc0e247c4
("Document ls-files -t as semi-obsolete.", 2010-07-28) because:

    "git ls-files -t" is [...] badly documented, hence we point the
    users to superior alternatives.
    The feature is marked as "semi-obsolete" but not "scheduled for removal"
    since it's a plumbing command, scripts might use it, and Git testsuite
    already uses it to test the state of the index.

Marking it as obsolete because it was easily misunderstood, which I
think was primarily due to documentation problems, is one strategy, but
I think fixing the documentation is a better option.  Especially since
in the intervening time, "git ls-files -t" has become heavily used by
sparse-checkout users where the same confusion just doesn't apply.

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