The stash.useBuiltin variable introduced in 90a462725e ("stash:
optionally use the scripted version again", 2019-02-25) was turned on by
default, but had no documentation.
Let's document it so that users who run into any stability issues with
the C rewrite know there's an escape hatch, and spell out that the
user should please report the bug when they have to turn off the
built-in stash.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
