Clarify and update GN build flags docs.

build/config/crypto.gni

 # Copyright (c) 2013 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 declares build flags for the SSL library configuration.
 #
 # TODO(brettw) this should probably be moved to src/crypto or somewhere, and
 # the global build dependency on it should be removed.
+#
+# 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() {
   # Use OpenSSL instead of NSS. This is used for Android, Mac, NaCl untrusted
   # code, and Windows, and is experimental in other cases (see
   # http://crbug.com/62803).
   use_openssl = is_android || is_mac || is_nacl || is_win
 }
 
 # True when we're using OpenSSL for representing certificates. When targeting
 # Android, the platform certificate library is used for certificate
 # verification. On other targets, this flag also enables OpenSSL for certificate
 # verification, but this configuration is unsupported.
 use_openssl_certs = is_android
 
 # True if NSS is used for certificate verification. Note that this is
 # independent from use_openssl. It is possible to use OpenSSL for the crypto
 # library, but NSS for the platform certificate library.
 use_nss_certs = is_linux