Update openapi-spec.md

Author: lindazqli

articles/ai-services/agents/how-to/tools/openapi-spec.md

@@ -37,6 +37,7 @@ work together, generate client code, create tests, apply design standards, and m
 1. Ensure you've completed the prerequisites and setup steps in the [quickstart](../../quickstart.md).
 
 1. Check the OpenAPI spec for the following requirements:
+    1. Though not required by OpenAPI Spec, `operationId` is required for each function to be used with OpenAPI tool.
     1. `operationId` should only contain letters, `-` and `_`. You can modify it to meet the requirement. We recommend using descriptive name to help models efficiently decide which function to use.
 
 ## Authenticating with API Key