assistant: Update comments

Bug: none
Test: NA
Change-Id: I4f577c8383408f8e716e52f96cf977af17b96697
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2137913Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758241}

chromeos/services/assistant/public/mojom/assistant.mojom

@@ -69,6 +69,9 @@ interface AssistantController {
 };
 
 // Interface to communicate with assistant backend.
+// The Assistant instance is held by chromeos::assistant::Service, which is
+// hosted in the browser process.
+// The callers are mostly in ash, and some are in autotest.
 interface Assistant {
   // Starts an interaction to edit the reminder uniquely identified by
   // |client_id|. In response to the request, LibAssistant will initiate
@@ -79,6 +82,7 @@ interface Assistant {
   // Starts a screen context interaction. Results related to the screen context
   // will be returned through the |AssistantInteractionSubscriber| interface to
   // registered subscribers.
+  // |assistant_screenshot| contains JPEG data.
   StartScreenContextInteraction(
       ax.mojom.AssistantStructure? assistant_structure,
       array<uint8> assistant_screenshot);