example-bot: add TODO for expect_problems

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

examples/echo_client.py

@@ -156,6 +156,7 @@ class EchoBot(ClientXMPP):
                 # `encrypt_message` excepts the plaintext to be sent, a list of
                 # bare JIDs to encrypt to, and optionally a dict of problems to
                 # expect per bare JID.
+                # TODO: Document expect_problems
                 recipients = [mto]
                 encrypt = await self['xep_0384'].encrypt_message(body, recipients)
                 msg.append(encrypt)