Do not create cross-ref links to undocumented members

[LearnCocos2D]

Following interface fragment will cause the "execute" word to generate a link to the undocumented method execute:

/**
 *  Initializes the action
 *
 *  @param block Block to execute.
 *
 *  @return An initialized call block action.
 */
- (id)initWithBlock:(void(^)())block;

// Executes the selector on the specific target.
- (void)execute;

Since execute is not documented appledoc should refrain from linking to it.