Clarify and update GN build flags docs.

build/config/features.gni

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# This file contains UI-related build flags. It should theoretically be in the
-# src/ui directory and only things that depend on the ui module should get the
-# definitions.
+# This file contains Chrome-feature-related build flags (see ui.gni for
+# UI-related ones). These should theoretically be moved to the build files of
+# the features themselves.
 #
 # However, today we have many "bad" dependencies on some of these flags from,
-# e.g. base, so they need to be global.
+# e.g. base, so they need to be global to match the GYP configuration. Also,
+# anything that needs a grit define must be in either this file or ui.gni.
 #
-# See also build/config/ui.gni
+# PLEASE TRY TO AVOID ADDING FLAGS TO THIS FILE in cases where grit isn't
+# required. See the declare_args block of BUILDCONFIG.gn for advice on how
+# to set up feature flags.
 
 import("//build/config/chrome_build.gni")
 if (is_android) {
   import("//build/config/android/config.gni")
 }
 
 declare_args() {
   # Multicast DNS.
   enable_mdns = is_win || is_linux
 
@@ skipping 164 matching lines @@
 # TODO(GYP) also require !embedded to enable.
 use_gconf = is_linux && !is_chromeos
 
 # Hangout services is an extension that adds extra features to Hangouts.
 # For official GYP builds, this flag is set, it will likely need to be
 # parameterized in the future for a similar use.
 enable_hangout_services_extension = false
 
 # Whether to back up data before sync.
 enable_pre_sync_backup = is_win || is_mac || (is_linux && !is_chromeos)