git stash: document apply's --index switch

Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-stash.txt

@@ -63,7 +63,7 @@ show [<stash>]::
 	it will accept any format known to `git-diff` (e.g., `git-stash show
 	-p stash@\{1}` to view the second most recent stash in patch form).
 
-apply [<stash>]::
+apply [--index] [<stash>]::
 
 	Restore the changes recorded in the stash on top of the current
 	working tree state.  When no `<stash>` is given, applies the latest
@@ -71,6 +71,11 @@ apply [<stash>]::
 +
 This operation can fail with conflicts; you need to resolve them
 by hand in the working tree.
++
+If the `--index` option is used, then tries to reinstate not only the working
+tree's changes, but also the index's ones. However, this can fail, when you
+have conflicts (which are stored in the index, where you therefore can no
+longer apply the changes as they were originally).
 
 clear::
 	Remove all the stashed states. Note that those states will then