Update documentation in //components/signin/core/browser/identity_utils.h

Rewrite the header as per the after-landing suggestions in CL1346093.

Bug: 907995
Change-Id: Iaecf3cd6257846f2012c7b42b97ffb844f4b45d7
Reviewed-on: https://chromium-review.googlesource.com/c/1349250Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Cr-Commit-Position: refs/heads/master@{#610557}

components/signin/core/browser/identity_utils.h

@@ -2,9 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
-// Collection of utility functions to support migrating different consumers of
-// the SigninManager component to the Identity service, which will eventually be
-// migrated to //services/identity once the SigninManager class gets removed.
+// Functions that are shared between the Identity Service implementation and its
+// consumers. Currently in //components/signin because they are used by classes
+// in this component, which cannot depend on //services/identity to avoid a
+// dependency cycle. When these classes have no direct consumers and are moved
+// to //services/identity, these functions should correspondingly be moved to
+// //services/identity/public/cpp.
 
 #ifndef COMPONENTS_SIGNIN_CORE_BROWSER_IDENTITY_UTILS_H_
 #define COMPONENTS_SIGNIN_CORE_BROWSER_IDENTITY_UTILS_H_
@@ -14,6 +17,9 @@
 namespace identity {
 
 // Returns true if the username is allowed based on the pattern string.
+//
+// NOTE: Can be moved to //services/identity/public/cpp once SigninManager is
+// moved to //services/identity.
 bool IsUsernameAllowedByPattern(base::StringPiece username,
                                 base::StringPiece pattern);
 }  // namespace identity