Update OpenAPI generator to add param and field documentation (#896)

We desperately need to document our APIs. This is the basic requirement
of having a Spec :)

This PR updates the OpenAPI generator so documentation for request
parameters and object fields can be properly added to the OpenAPI specs.
From there, this should get picked by Stainless, etc.

## Test Plan:

Updated client-sdk (See
https://github.com/meta-llama/llama-stack-client-python/pull/104) and
then ran:

```bash
cd tests/client-sdk
LLAMA_STACK_CONFIG=../../llama_stack/templates/fireworks/run.yaml pytest -s -v inference/test_inference.py agents/test_agents.py
```

llama_stack/apis/agents/agents.py

@@ -297,6 +297,16 @@ class AgentStepResponse(BaseModel):
 @runtime_checkable
 @trace_protocol
 class Agents(Protocol):
+    """Agents API for creating and interacting with agentic systems.
+
+    Main functionalities provided by this API:
+    - Create agents with specific instructions and ability to use tools.
+    - Interactions with agents are grouped into sessions ("threads"), and each interaction is called a "turn".
+    - Agents can be provided with various tools (see the ToolGroups and ToolRuntime APIs for more details).
+    - Agents can be provided with various shields (see the Safety API for more details).
+    - Agents can also use Memory to retrieve information from knowledge bases. See the RAG Tool and Vector IO APIs for more details.
+    """
+
     @webmethod(route="/agents", method="POST")
     async def create_agent(
         self,