Add 'agent_id' input field to intent_conversation_api.md file (#2520)

As the `api/conversation/process` endpoint uses the (conversation action)[https://www.home-assistant.io/integrations/conversation/], the `agent_id` can be used and must be documented.

Author: vaites

docs/intent_conversation_api.md

@@ -34,6 +34,7 @@ The following input fields are available:
 |-------------------|--------|---------------------------------------------------------------------------------------------|
 | `text`            | string | Input sentence.                                                                             |
 | `language`        | string | Optional. Language of the input sentence (defaults to configured language).                 |
+| `agent_id`        | string | Optional. Conversation agent to process the request (defaults _home_assistant_).            |
 | `conversation_id` | string | Optional. Unique id to [track conversation](#conversation-id). Generated by Home Assistant. |