lightningd: make sure rpc_command replacement is well-formed.

In particular:
1. It must redirect to an existing command.
2. It must contain method, params and id.

And update the docs to show the id, which is vital.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_plugin.py

@@ -761,7 +761,6 @@ def test_sendpay_notifications(node_factory, bitcoind):
     assert results['sendpay_failure'][0] == err.value.error
 
 
-@pytest.mark.xfail(strict=True)
 def test_rpc_command_hook(node_factory):
     """Test the `sensitive_command` hook"""
     plugin = os.path.join(os.getcwd(), "tests/plugins/rpc_command.py")