[PATCH 2/2] update-index doc: note the caveat with "could not open..."

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

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>
---
 Documentation/git-update-index.txt | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/Documentation/git-update-index.txt b/Documentation/git-update-index.txt
index e30b185918..0c81600d8c 100644
--- a/Documentation/git-update-index.txt
+++ b/Documentation/git-update-index.txt
@@ -480,6 +480,16 @@ a directory with a file when it comes to the internal structures of
 the untracked cache, but no case has been found where this resulted in
 wrong "git status" output.
 
+There are also cases where existing indexes written by git versions
+before 2.17 will reference directories that don't exist anymore,
+potentially causing many "could not open directory" warnings to be
+printed on "git status". These are new warnings for existing issues
+that were previously silently discarded.
+
+As with the bug described above the solution is to one-off do a "git
+status" run with `core.untrackedCache=false` to flush out the leftover
+bad data.
+
 File System Monitor
 -------------------
 
-- 
2.15.1.424.g9478a66081