Clarify and update GN build flags docs.

build/config/ui.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 UI-related build flags (see features.gni for Chrome
+# feature-related ones). These should theoretically be moved to the ui
+# directory.
 #
 # 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 features.gni.
 #
-# See also build/config/features.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.
 
 declare_args() {
   # Indicates if Ash is enabled. Ash is the Aura Shell which provides a
   # desktop-like environment for Aura. Requires use_aura = true
   use_ash = is_win || is_linux
 
   # Indicates if Ozone is enabled. Ozone is a low-level library layer for Linux
   # that does not require X11.
   use_ozone = false
 
@@ skipping 33 matching lines @@
 use_default_render_theme = use_aura || is_linux
 
 # Indicates if the UI toolkit depends on X11.
 use_x11 = is_linux && !use_ozone
 
 use_ozone_evdev = use_ozone
 
 use_clipboard_aurax11 = is_linux && use_aura && use_x11
 
 enable_hidpi = is_mac || is_chromeos || is_win || is_linux