rebase doc: document rebase.useBuiltin

rebase doc: document rebase.useBuiltin

The rebase.useBuiltin variable introduced in 55071ea248 ("rebase:
start implementing it as a builtin", 2018-08-07) was turned on by
default in 5541bd5b8f ("rebase: default to using the builtin rebase",
2018-08-08), 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[1], and make it clear that
needing to turn off builtin rebase means you've found a bug in git.

1. https://public-inbox.org/git/87y39w1wc2.fsf@evledraar.gmail.com/

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