pyln-client: document and test that init can self-disable.

mypy says it returns None, but it actually doesn't have to!

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

tests/plugins/selfdisable.py

@@ -0,0 +1,12 @@
+#!/usr/bin/env python3
+from pyln.client import Plugin
+
+plugin = Plugin()
+
+
+@plugin.init()
+def init(configuration, options, plugin):
+    return {'disable': 'init saying disable'}
+
+
+plugin.run()