Add more gn documentation

tools/gn/function_set_default_toolchain.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/build_settings.h"
 #include "tools/gn/functions.h"
 #include "tools/gn/loader.h"
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/scope.h"
 #include "tools/gn/settings.h"
@@ skipping 23 matching lines @@
   config file for the 64-bit toolchain to reset the default toolchain to
   64-bit, we want to keep it 32-bits.
 
 Argument
 
   toolchain_label
       Toolchain name.
 
 Example
 
-  set_default_toolchain("//build/config/win:vs32"))";
+  # Set default toolchain only has an effect when run in the context of the
+  # default toolchain. Pick the right one according to the current CPU
+  # architecture.
+  if (target_cpu == "x64") {
+    set_default_toolchain("//toolchains:64")
+  } else if (target_cpu == "x86") {
+    set_default_toolchain("//toolchains:32")
+  }
+)";
 
 Value RunSetDefaultToolchain(Scope* scope,
                              const FunctionCallNode* function,
                              const std::vector<Value>& args,
                              Err* err) {
   if (!scope->IsProcessingBuildConfig()) {
     *err = Err(function->function(), "Must be called from build config.",
         "set_default_toolchain can only be called from the build configuration "
         "file.");
     return Value();
@@ skipping 14 matching lines @@
   Label toolchain_label(
       Label::Resolve(current_dir, default_toolchain, args[0], err));
   if (toolchain_label.is_null())
     return Value();
 
   *default_toolchain_dest = toolchain_label;
   return Value();
 }
 
 }  // namespace functions