[ast] Annotate some nodes with number of yields.

src/ast/ast-numbering.h

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_AST_AST_NUMBERING_H_
 #define V8_AST_AST_NUMBERING_H_
 
 namespace v8 {
 namespace internal {
 
 // Forward declarations.
 class FunctionLiteral;
 class Isolate;
 class Zone;
 
 namespace AstNumbering {
-// Assign type feedback IDs and bailout IDs to an AST node tree.
-//
+// Assign type feedback IDs and bailout IDs to an AST node tree.  For a
+// generator function, also annotate the function itself and any loops therein
+// with the number of contained yields.
 bool Renumber(Isolate* isolate, Zone* zone, FunctionLiteral* function);
 }
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_AST_AST_NUMBERING_H_