update-index doc: note a fixed bug in the untracked cache

update-index doc: note a fixed bug in the untracked cache

Document the bug tested for in my "status: add a failing test showing
a core.untrackedCache bug" and fixed in Duy's "dir.c: fix missing dir
invalidation in untracked code".

Since this is very likely something others will encounter in the
future on older versions, and it's not obvious how to fix it let's
document both that it exists, and how to "fix" it with a one-off
command.

As noted in that commit, even though this bug gets the untracked cache
into a bad state, we have not yet found a case where this is user
visible, and thus it makes sense for these docs to focus on the
symlink case only.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>