Add quick reference for bind/callback usage.

base/bind.h

Index: base/bind.h
diff --git a/base/bind.h b/base/bind.h
index 22a3b4b83edacd35bda0bd3294c57e0c08512ae2..a86e9568604e9dbfc582663b94741a4666cb1f6b 100644
--- a/base/bind.h
+++ b/base/bind.h
@@ -14,12 +14,23 @@
 #include "base/bind_internal.h"
 #include "base/callback_internal.h"
 
-// See base/callback.h for how to use these functions. If reading the
-// implementation, before proceeding further, you should read the top
-// comment of base/bind_internal.h for a definition of common terms and
-// concepts.
+// -----------------------------------------------------------------------------
+// Usage documentation
+// -----------------------------------------------------------------------------
+//
+// See base/callback.h for documentation.
+//
+//
+// -----------------------------------------------------------------------------
+// Implementation notes
+// -----------------------------------------------------------------------------
+//
+// If you're reading the implementation, before proceeding further, you should
+// read the top comment of base/bind_internal.h for a definition of common
+// terms and concepts.
+//
+// RETURN TYPES
 //
-// IMPLEMENTATION NOTE
 // Though Bind()'s result is meant to be stored in a Callback<> type, it
 // cannot actually return the exact type without requiring a large amount
 // of extra template specializations. The problem is that in order to