Clarify and update GN build flags docs.

build/config/features.gni

Index: build/config/features.gni
diff --git a/build/config/features.gni b/build/config/features.gni
index 82b7d066fddf0cb180a073ac1ded0f3f1ca30a7a..f362777e92eb6974350f7e1881b4cc824303fb96 100644
--- a/build/config/features.gni
+++ b/build/config/features.gni
@@ -2,14 +2,17 @@
 # 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) {