Add missing docstring

homeassistant/components/mqtt/discovery.py

@@ -129,6 +129,7 @@ async def async_device_message_received(topic, payload, qos):
             pattern = r'(?:(?<=^)|(?<=_))[^\W_]+(?=_|$)'
 
             def expand(matchobj):
+            """Helper to expand a possibly abbreviated word."""
                 abbreviation = matchobj.group(0)
                 if abbreviation in ABBREVIATIONS:
                     return ABBREVIATIONS[abbreviation]