stash: document stash.useBuiltin

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>

Documentation/config/stash.txt

@@ -1,3 +1,18 @@
+stash.useBuiltin::
+       Set to `false` to use the legacy shell script implementation of
+       linkgit:git-stash[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 2.22 (and Git for Windows
+version 2.19). 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 shell script version of linkgit:git-stash[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 (see https://git-scm.com/community for details).
+
 stash.showPatch::
 	If this is set to true, the `git stash show` command without an
 	option will show the stash entry in patch form.  Defaults to false.