message: Update ir.md to clarify markdown is allowed for doc_strings
diff: @@ -34,7 +34,7 @@ The following are the metadata properties of a model graph:
  |name|string|Valid C identifier|A name for the model.|
  |domain|string|Valid DNS name|A namespace for the model, following the style of package names, that is, reverse DNS domain name.|
  |ir_version|int64||The version of the IR format specification|
  -|doc_string|string|Free form|A human-readable documentation string intended to summarize the purpose of the model.|
  +|doc_string|string|Free form|A human-readable documentation string intended to summarize the purpose of the model. Markdown is allowed.|
  #### Names Within a Graph
  