Add quick reference for bind/callback usage.

base/bind_internal.h.pump

 $$ This is a pump file for generating file templates.  Pump is a python
 $$ script that is part of the Google Test suite of utilities.  Description
 $$ can be found here:
 $$
 $$ http://code.google.com/p/googletest/wiki/PumpManual
 $$
 
 $$ See comment for MAX_ARITY in base/bind.h.pump.
 $var MAX_ARITY = 7
 $range ARITY 0..MAX_ARITY
@@ skipping 13 matching lines @@
 #include "base/template_util.h"
 #include "build/build_config.h"
 
 #if defined(OS_WIN)
 #include "base/bind_internal_win.h"
 #endif
 
 namespace base {
 namespace internal {
 
+// See base/callback.h for user documentation.
+//
+//
 // CONCEPTS:
 //  Runnable -- A type (really a type class) that has a single Run() method
 //              and a RunType typedef that corresponds to the type of Run().
 //              A Runnable can declare that it should treated like a method
 //              call by including a typedef named IsMethod.  The value of
 //              this typedef is NOT inspected, only the existence.  When a
 //              Runnable declares itself a method, Bind() will enforce special
 //              refcounting + WeakPtr handling semantics for the first
 //              parameter which is expected to be an object.
 //  Functor -- A copyable type representing something that should be called.
@@ skipping 441 matching lines @@
 
 ]]
 };
 
 ]] $$ for ARITY
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_BIND_INTERNAL_H_