Add comments about threadSafeBind()

Source/platform/ThreadSafeFunctional.h

Index: Source/platform/ThreadSafeFunctional.h
diff --git a/Source/platform/ThreadSafeFunctional.h b/Source/platform/ThreadSafeFunctional.h
index 1b5b5dac85d625b4947fbf5d911a19e06dd3e201..9b8efdbad5274fcc71b49d9880d2149455a7ede7 100644
--- a/Source/platform/ThreadSafeFunctional.h
+++ b/Source/platform/ThreadSafeFunctional.h
@@ -10,6 +10,22 @@
 
 namespace blink {
 
+// threadSafeBind() is bind() for cross-thread task posting.
+// threadSafeBind() applies CrossThreadCopier to the arguments.
+//
+// Example:
+//     void func1(int, const String&);
+//     f = threadSafeBind(func1, 42, str);
+// func1(42, str2) will be called when |f()| is executed,
+// where |str2| is a deep copy of |str| (created by str.isolatedCopy()).
+//
+// threadSafeBind(str) is similar to bind(str.isolatedCopy()), but the latter
+// is NOT thread-safe due to temporary objects (https://crbug.com/390851).
+//
+// Don't (if you pass the task across threads):
+//     bind(func1, 42, str);
+//     bind(func1, 42, str.isolatedCopy());
+
 template<typename... FreeVariableTypes, typename FunctionType>
 PassOwnPtr<Function<typename WTF::FunctionWrapper<FunctionType>::ResultType(FreeVariableTypes...)>> threadSafeBind(
     FunctionType function)