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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Documentation/git-archive.txt

@@ -65,7 +65,10 @@ OPTIONS
 
 --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

Documentation/git-upload-archive.txt

@@ -20,6 +20,32 @@ This command is usually not invoked directly by the end user.  The UI
 for the protocol is on the 'git archive' side, and the program pair
 is meant to be used to get an archive from a remote repository.
 
+SECURITY
+--------
+
+In order to protect the privacy of objects that have been removed from
+history but may not yet have been pruned, `git-upload-archive` avoids
+serving archives for commits and trees that are not reachable from the
+repository's refs.  However, because calculating object reachability is
+computationally expensive, `git-upload-archive` implements a stricter
+but easier-to-check set of rules:
+
+  1. Clients may request a commit or tree that is pointed to directly by
+     a ref. E.g., `git archive --remote=origin v1.0`.
+
+  2. Clients may request a sub-tree within a commit or tree using the
+     `ref:path` syntax. E.g., `git archive --remote=origin v1.0:Documentation`.
+
+  3. Clients may _not_ use other sha1 expressions, even if the end
+     result is reachable. E.g., neither a relative commit like `master^`
+     nor a literal sha1 like `abcd1234` is allowed, even if the result
+     is reachable from the refs.
+
+Note that rule 3 disallows many cases that do not have any privacy
+implications. These rules are subject to change in future versions of
+git, and the server accessed by `git archive --remote` may or may not
+follow these exact rules.
+
 OPTIONS
 -------
 <directory>::