docs: clarify remote restrictions for git-upload-archive

Commits ee27ca4 and 0f544ee introduced rules by which
git-upload-archive would restrict clients from accessing
unreachable objects. However, we never documented those
rules anywhere, nor their reason for being. Let's do so now.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-archive.txt b/Documentation/git-archive.txt
index b97aaab..cfa1e4e 100644
--- a/Documentation/git-archive.txt
+++ b/Documentation/git-archive.txt
@@ -65,7 +65,10 @@
 
 --remote=<repo>::
 	Instead of making a tar archive from the local repository,
-	retrieve a tar archive from a remote repository.
+	retrieve a tar archive from a remote repository. Note that the
+	remote repository may place restrictions on which sha1
+	expressions may be allowed in `<tree-ish>`. See
+	linkgit:git-upload-archive[1] for details.
 
 --exec=<git-upload-archive>::
 	Used with --remote to specify the path to the