Document binding behavior for C call/return events for TracePoint/set_trace_func

C methods do not have bindings, so binding returns the binding of
the nearest C method.

Fixes [Bug #9009]

trace_point.rb

@@ -309,15 +309,21 @@ class TracePoint
     Primitive.tracepoint_attr_defined_class
   end
 
-  # Return the generated binding object from event
+  # Return the generated binding object from event.
+  #
+  # Note that for +c_call+ and +c_return+ events, the binding returned is the
+  # binding of the nearest Ruby method calling the C method, since C methods
+  # themselves do not have bindings.
   def binding
     Primitive.tracepoint_attr_binding
   end
 
   # Return the trace object during event
   #
-  # Same as TracePoint#binding:
-  #	trace.binding.eval('self')
+  # Same as the following, except it returns the correct object (the method
+  # receiver) for +c_call+ and +c_return+ events:
+  #
+  #   trace.binding.eval('self')
   def self
     Primitive.tracepoint_attr_self
   end

vm_trace.c

@@ -519,6 +519,10 @@ static void call_trace_func(rb_event_flag_t, VALUE data, VALUE self, ID id, VALU
  *	  line prog.rb:3        test     Test
  *	  line prog.rb:4        test     Test
  *      return prog.rb:4        test     Test
+ *
+ * Note that for +c-call+ and +c-return+ events, the binding returned is the
+ * binding of the nearest Ruby method calling the C method, since C methods
+ * themselves do not have bindings.
  */
 
 static VALUE