Add back comments about the requirements for attribute naming that were originally on the AttributeDescriptor message

Author: ZackButcher

mixer/v1/config/cfg.proto

@@ -153,8 +153,18 @@ message AttributeManifest {
     istio.mixer.v1.config.descriptor.ValueType value_type = 2;
   }
   // The set of attributes this Istio component will be responsible for producing at runtime.
-  // We map from attribute name to the attribute's specification. Attribute names must be
-  // unique across all manifests in the configuration.
+  // We map from attribute name to the attribute's specification. The name of an attribute,
+  // which is how attributes are referred to in aspect configuration, must conform to:   
+  // 
+  //     Name = IDENT { SEPARATOR IDENT };
+  // 
+  // Where `IDENT` must match the regular expression `[a-z][a-z0-9]+` and `SEPARATOR` must
+  // match the regular expression `[.-]`.
+  // 
+  // Attribute names must be unique within a single Istio deployment. The set of canonical
+  // attributes are described at https://istio.io/docs/reference/attribute-vocabulary.html.
+  // Attributes not in that list should be named with a component-specific suffix such as
+  // request.count-my.component
   map<string, AttributeInfo> attributes = 3;
 }