[FTP/FTPS Only] Completes the asynchronous operation started in BeginGetCommandHelp.
Help text from the server for the supplied command.
Before calling BeginGetCommandHelp, you need to create a callback method that implements the AsyncCallback delegate. This callback method executes in a separate thread and is called by the system after BeginGetCommandHelp returns. The callback method must accept the IAsyncResult returned by the BeginGetCommandHelp method as a parameter. This parameter may then be passed to the EndGetCommandHelp method to complete the operation.