In 8a2cd3f512 (stash: remove the stash.useBuiltin setting, 2020-03-03),
we removed the setting, and for a couple of major versions, we still
documented the setting, telling users that it is gone.
We can now safely remove even the documentation.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>