[identity] Update comments about IdentityManager

Update comments in //services/identity/public/cpp to better reflect the
current reality of IdentityManager.

Change-Id: I20fef4d73aec6737927bdc0dc398dd366fccbd9a
Reviewed-on: https://chromium-review.googlesource.com/1013479Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551136}

services/identity/public/cpp/README.md

-IdentityManager serves as the primary client-side interface to the Identity
-Service, encapsulating a connection to a remote implementation of
-identity::mojom::IdentityManager. It provides conveniences over the bare Mojo
-interfaces such as:
+IdentityManager is the next-generation C++ API for interacting with Google
+identity. It is currently backed by //components/signin (see IMPLEMENTATION
+NOTES below); in the long-term it will serve as the primary client-side
+interface to the Identity Service, encapsulating a connection to a remote
+implementation of identity::mojom::IdentityManager. It provides conveniences
+over the bare Identity Service Mojo interfaces such as:
 
 - Synchronous access to the information of the primary account (via caching)
 

services/identity/public/cpp/identity_manager.h

@@ -26,8 +26,7 @@ class ProfileSyncServiceHarness;
 
 namespace identity {
 
-// Primary client-side interface to the Identity Service, encapsulating a
-// connection to a remote implementation of mojom::IdentityManager. See
+// Gives access to information about the user's Google identities. See
 // ./README.md for detailed documentation.
 class IdentityManager : public SigninManagerBase::Observer,
 #if !defined(OS_CHROMEOS)