Add a link to python's logging config schema (#5926)

changelog.d/5926.misc

+Add link in sample config to the logging config schema.

docs/sample_config.yaml

@@ -485,7 +485,8 @@ database:
 
 ## Logging ##
 
-# A yaml python logging config file
+# A yaml python logging config file as described by
+# https://docs.python.org/3.7/library/logging.config.html#configuration-dictionary-schema
 #
 log_config: "CONFDIR/SERVERNAME.log.config"
 

synapse/config/logger.py

@@ -89,7 +89,8 @@ class LoggingConfig(Config):
             """\
         ## Logging ##
 
-        # A yaml python logging config file
+        # A yaml python logging config file as described by
+        # https://docs.python.org/3.7/library/logging.config.html#configuration-dictionary-schema
         #
         log_config: "%(log_config)s"
         """