message: Clarify that Markdown is allowed for doc_strings
diff: @@ -76,7 +76,7 @@ message NodeProto {
  // Additional named attributes.
  repeated AttributeProto attribute = 5;
  - // A human-readable documentation for this graph.
  + // A human-readable documentation for this graph. Markdown is allowed.
  optional string doc_string = 6;
  }
  @@ -121,7 +121,7 @@ message GraphProto {
  // the graph.
  optional string domain = 9;
  - // A human-readable documentation for this graph.
  + // A human-readable documentation for this graph. Markdown is allowed.
  optional string doc_string = 10;
  };
  