update-index doc: note the caveat with "could not open..."

Note the caveat where 2.17 is stricter about index validation
potentially causing "could not open directory" warnings when git is
upgraded. See the preceding "dir.c: stop ignoring opendir() error in
open_cached_dir()" change.

This caused some mayhem when I upgraded git to a version with this
series at Booking.com, and other users have doubtless enabled the UC
extension and are in for a surprise when they upgrade. Let's give them
a headsup in the docs.

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

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