[Docs] Document incompatible changes to :shutdown command

The :shutdown command is used to shutdown a remote server. The common case is that a user specifies the impalad to shutdown by specifying a host e.g. :shutdown('host100'). If a user has more than one impalad on a remote host then the form :shutdown('<host>:<port>') can be used to specify the port by which the imapald can be contacted. Prior to IMPALA-7985 this port was the backend port, e.g. :shutdown('host100:22000'). With IMPALA-7985 the port to use is the KRPC port , e.g. :shutdown('host100:27000').