docs: add documentation for RAGDocument (#1693)

# What does this PR do?


## Test Plan

docs/_static/llama-stack-spec.yaml

@@ -5375,6 +5375,7 @@ components:
       properties:
         document_id:
           type: string
+          description: The unique identifier for the document.
         content:
           oneOf:
             - type: string
@@ -5383,8 +5384,10 @@ components:
               items:
                 $ref: '#/components/schemas/InterleavedContentItem'
             - $ref: '#/components/schemas/URL'
+          description: The content of the document.
         mime_type:
           type: string
+          description: The MIME type of the document.
         metadata:
           type: object
           additionalProperties:
@@ -5395,12 +5398,15 @@ components:
               - type: string
               - type: array
               - type: object
+          description: Additional metadata for the document.
       additionalProperties: false
       required:
         - document_id
         - content
         - metadata
       title: RAGDocument
+      description: >-
+        A document to be used for document ingestion in the RAG Tool.
     InsertRequest:
       type: object
       properties: