message: Add OperatorSetId definition in docs
  * Add OperatorSetId definition in docs
  * Extend OperatorSetId definition
  Add additional information about version and domain fields.
diff: @@ -114,6 +114,18 @@ Name|Type|Format|Description
  model_author|string|A comma-separated list of names.|The personal name of the author(s) of the model, and/or their organizations.
  model_license|string|Name or URL.|The well-known name or URL of the license under which the model is made available.
  +### Operator Set Identifiers
  +
  +Each operator set is uniquely identified by a (domain, version) pair.
  +
  +Name|Type|Description
  +|---|---|---|
  +domain|string|The domain of the operator set being identified.
  +version|int64|The version of the operator set being identified. Same as 'opset_version' in the operator set.
  +
  +The operator set version is a simple integer value that is monotonically increased as new versions of the operator set are published.
  +
  +Operator sets other than the default operator set MUST specify a domain and SHOULD use reverse domain names based on the responsible organization's identity, the same convention that is used for [naming Java packages](https://docs.oracle.com/javase/tutorial/java/package/namingpkgs.html).
  ### Operator Sets
  