docs: update docstrings for better formatting (#3838)

# What does this PR do?
Updates docstrings for Conversations and Eval APIs to render better in
the docs nav sidebar.

Before: 
<img width="363" height="233" alt="Screenshot 2025-10-17 at 9 52 17 AM"
src="https://github.com/user-attachments/assets/3a77f9e3-3b03-43ae-8584-a21d1f44d54d"
/>

After:
<img width="410" height="206" alt="Screenshot 2025-10-17 at 9 52 11 AM"
src="https://github.com/user-attachments/assets/fa5d428d-2bde-4453-84fd-9aceebe712e8"
/>


## Test Plan
* Manual testing

docs/static/deprecated-llama-stack-spec.yaml

@@ -10196,9 +10196,9 @@ tags:
   - name: Datasets
     description: ''
   - name: Eval
-    description: ''
-    x-displayName: >-
+    description: >-
       Llama Stack Evaluation API for running evaluations on model and agent candidates.
+    x-displayName: Evaluations
   - name: Files
     description: >-
       This API is used to upload documents that can be used with other Llama Stack