doc: explain the impact of stash.index on --autostash options

With 9842c0c (stash: honor stash.index in apply, pop modes,
2025-09-21) merged in a5d4779 (Merge branch 'dk/stash-apply-index',
2025-09-29), we did not advertise the connection between the new config
option stash.index and the implicit use of git-stash via --autostash
(which may also be configured). Do so.

Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: benknoble

Documentation/config/stash.adoc

@@ -2,6 +2,10 @@ stash.index::
 	If this is set to true, `git stash apply` and `git stash pop` will
 	behave as if `--index` was supplied. Defaults to false. See the
 	descriptions in linkgit:git-stash[1].
++
+This also affects invocations of linkgit:git-stash[1] via `--autostash` from
+commands like linkgit:git-merge[1], linkgit:git-rebase[1], and
+linkgit:git-pull[1].
 
 stash.showIncludeUntracked::
 	If this is set to true, the `git stash show` command will show