Update service_manager::Identity documentation on |user_id|

The comment is copied from the "Service Manager User Guide" in
services/service_manager/README.md.

Change-Id: Ia5b49371366162465a61e99c9ce6a0dad094f6fd
Reviewed-on: https://chromium-review.googlesource.com/673142Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502952}

services/service_manager/public/cpp/identity.h

@@ -13,6 +13,8 @@ namespace service_manager {
 
 // Represents the identity of an application.
 // |name| is the structured name of the application.
+// |user_id| is a GUID string representing the identity of a user in the system.
+// Every running service instance is associated with a specific user ID.
 // |instance| is a string that allows to tie a specific instance to another. A
 // typical use case of instance is to control process grouping for a given name.
 class SERVICE_MANAGER_PUBLIC_CPP_TYPES_EXPORT Identity {