title: What is the use of privdata argument in redisAsyncCommand?
body: <p>The function definition provided in the source for redisAsyncCommand( ) is:</p>
  
  <p><code>static int __redisAsyncCommand(redisAsyncContext *ac, redisCallbackFn *fn, void *privdata, const char *cmd, size_t len) { ... }</code></p>
  
  <p>What is the purpose of the <code>void *privdata</code> argument? In what cases would it be useful?</p>