Add Comment to __init__()

Author: fillbit

config/exec_provider.py

@@ -32,6 +32,10 @@ class ExecProvider(object):
     """
 
     def __init__(self, exec_config):
+        """
+        exec_config must be of type ConfigNode because we depend on safe_get(self, key)
+        to correctly handle optional exec provider config parameters.
+        """
         for key in ['command', 'apiVersion']:
             if key not in exec_config:
                 raise ConfigException(