Document how ssh-agent cannot be used for replication

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/Documentation/config-replication.txt b/Documentation/config-replication.txt
index 7d10b35..ceee0c1 100644
--- a/Documentation/config-replication.txt
+++ b/Documentation/config-replication.txt
@@ -80,7 +80,7 @@
 
 SSH authentication must be by passwordless public key, as there is
 no facility to read passphases on startup or passwords during the
-SSH connection setup.
+SSH connection setup, and SSH agents are not supported from Java.
 
 Host keys for any destination SSH servers must appear in the user's
 `~/.ssh/known_hosts` file, and must be added in advance, before