Add some more comments in NoAvailableSession handling

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

examples/echo_client.py

@@ -98,6 +98,9 @@ class EchoBot(ClientXMPP):
                 # We received a message from that contained a session that we
                 # don't know about (deleted session storage, etc.). We can't
                 # decrypt the message, and it's going to be lost.
+                # Here, as we need to initiate a new encrypted session, it is
+                # best if we send an encrypted message directly. XXX: Is it
+                # where we talk about self-healing messages?
                 self.encrypted_reply(
                     msg,
                     'I can\'t decrypt this message as it uses an encrypted '