refine doc about API: completion (#829)

### What problem does this PR solve?
#808 

### Type of change

- [x] Documentation Update

docs/conversation_api.md

 | name | type | optional | description|
 |------|-------|----|----|
 | conversation_id| string | No | This is from calling /new_conversation.|
-| messages| json | No | All the conversation history stored here including the latest user's question.|
+| messages| json | No | The latest question, such as `[{"role": "user", "content": "How are you doing!"}]`|
 | quote | bool | Yes | Default: true |
+| stream | bool | Yes | Default: true |
+| doc_ids | string | Yes | Document IDs which is delimited by comma, like `c790da40ea8911ee928e0242ac180005,c790da40ea8911ee928e0242ac180005`. The retrieved content is limited in these documents. |
 
 ### Response 
 ```json