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>

Documentation/config/rebase.txt

@@ -1,3 +1,17 @@
+rebase.useBuiltin::
+	Set to `false` to use the legacy shellscript implementation of
+	linkgit:git-rebase[1]. Is `true` by default, which means use
+	the built-in rewrite of it in C.
++
+The C rewrite is first included with Git version 2.20. This option
+serves an an escape hatch to re-enable the legacy version in case any
+bugs are found in the rewrite. This option and the shellscript version
+of linkgit:git-rebase[1] will be removed in some future release.
++
+If you find some reason to set this option to `false` other than
+one-off testing you should report the behavior difference as a bug in
+git.
+
 rebase.stat::
 	Whether to show a diffstat of what changed upstream since the last
 	rebase. False by default.