Update GN toolchain_args to be a variable.

tools/gn/args.cc

 // 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.
 
 #include "tools/gn/args.h"
 
 #include "base/sys_info.h"
 #include "build/build_config.h"
 #include "tools/gn/variables.h"
 
@@ skipping 17 matching lines @@
     "  If specified, arguments from the --args command line flag are used. If\n"
     "  that flag is not specified, args from previous builds in the build\n"
     "  directory will be used (this is in the file args.gn in the build\n"
     "  directory).\n"
     "\n"
     "  Last, for targets being compiled with a non-default toolchain, the\n"
     "  toolchain overrides are applied. These are specified in the\n"
     "  toolchain_args section of a toolchain definition. The use-case for\n"
     "  this is that a toolchain may be building code for a different\n"
     "  platform, and that it may want to always specify Posix, for example.\n"
-    "  See \"gn help toolchain_args\" for more.\n"
+    "  See \"gn help toolchain\" for more.\n"
     "\n"
     "  If you specify an override for a build argument that never appears in\n"
     "  a \"declare_args\" call, a nonfatal error will be displayed.\n"
     "\n"
     "Examples\n"
     "\n"
     "  gn args out/FooBar\n"
     "      Create the directory out/FooBar and open an editor. You would type\n"
     "      something like this into that file:\n"
     "          enable_doom_melon=false\n"
@@ skipping 299 matching lines @@
     Scope* scope) const {
   lock_.AssertAcquired();
   return declared_arguments_per_toolchain_[scope->settings()];
 }
 
 Scope::KeyValueMap& Args::OverridesForToolchainLocked(
     Scope* scope) const {
   lock_.AssertAcquired();
   return toolchain_overrides_[scope->settings()];
 }