Add docstrings to globalmessages data interface

endpoints/api/globalmessages_models_interface.py

@@ -23,17 +23,32 @@ class GlobalMessage(
 
 @add_metaclass(ABCMeta)
 class GlobalMessageDataInterface(object):
+  """
+  Data interface for globalmessages API
+  """
   
   @abstractmethod
   def get_all_messages(self):
-    pass
+    """
+    
+    Returns:
+    list(GlobalMessage)
+    """
   
   @abstractmethod
-  def create_message(self):
-    pass
+  def create_message(self, severity, media_type_name, content):
+    """
+    
+    Returns:
+    GlobalMessage or None
+    """
   
   @abstractmethod
-  def delete_message(self):
-    pass
+  def delete_message(self, uuid):
+    """
+    
+    Returns:
+    void
+    """