Add documentation for Crosapi/BrowserService.

BrowserService is always the wrong interface to use when code in ash
needs to communiate with Lacros. This CL updates the documentation to
indicate this.

Change-Id: I253252777051d784dc9159d9b0f5621a295c9d9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2782224
Commit-Queue: Erik Chen <erikchen@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Roland Bock <rbock@google.com>
Cr-Commit-Position: refs/heads/master@{#866814}

chromeos/crosapi/mojom/crosapi.mojom

@@ -373,8 +373,14 @@ struct BrowserInitParams {
 
 // BrowserService defines the APIs that live in a browser (such as
 // lacros-chrome) and are accessed from ash-chrome.
+// IMPORTANT NOTE: If you are exposing a new capability from Lacros to Ash,
+// create a new mojom file. See automation.mojom for an example. This interface
+// has accidentally become a kitchen sink for different features. This was not
+// intentional.
+//
 // Next MinVersion: 11.
 // Next ID: 9
+//
 [Stable, Uuid="4e04dc16-b34c-40fd-9e3f-3c55c2c6cf91",
  RenamedFrom="crosapi.mojom.LacrosChromeService"]
 interface BrowserService {