Add more gn documentation

tools/gn/function_set_default_toolchain.cc

Index: tools/gn/function_set_default_toolchain.cc
diff --git a/tools/gn/function_set_default_toolchain.cc b/tools/gn/function_set_default_toolchain.cc
index b9d31099cdb8b052a8fa13f8bd24dd2b5e4c6655..f04cba88031d73752174c483755585fcad4090a3 100644
--- a/tools/gn/function_set_default_toolchain.cc
+++ b/tools/gn/function_set_default_toolchain.cc
@@ -41,7 +41,15 @@ Argument
 
 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,