MINOR: Improve the help doc of consumer group command

Clarify the consumer group command help message around `zookeeper`, `bootstrap-server`, and `new-consumer` options.

Author: Vahid Hashemian <vahidhashemian@us.ibm.com>

Reviewers: Jason Gustafson <jason@confluent.io>

Closes #2046 from vahidhashemian/minor/improve_consumer_group_command_doc

core/src/main/scala/kafka/admin/ConsumerGroupCommand.scala

@@ -676,9 +676,9 @@ object ConsumerGroupCommand extends Logging {
   }
 
   class ConsumerGroupCommandOptions(args: Array[String]) {
-    val ZkConnectDoc = "REQUIRED (only when using old consumer): The connection string for the zookeeper connection in the form host:port. " +
+    val ZkConnectDoc = "REQUIRED (for consumer groups based on the old consumer): The connection string for the zookeeper connection in the form host:port. " +
       "Multiple URLS can be given to allow fail-over."
-    val BootstrapServerDoc = "REQUIRED (unless old consumer is used): The server to connect to."
+    val BootstrapServerDoc = "REQUIRED (for consumer groups based on the new consumer): The server to connect to."
     val GroupDoc = "The consumer group we wish to act on."
     val TopicDoc = "The topic whose consumer group information should be deleted or topic whose should be included in the reset offset process. " +
       "In `reset-offsets` case, partitions can be specified using this format: `topic1:0,1,2`, where 0,1,2 are the partition to be included in the process. " +
@@ -694,7 +694,7 @@ object ConsumerGroupCommand extends Logging {
       "Pass in just a topic to delete the given topic's partition offsets and ownership information " +
       "for every consumer group. For instance --topic t1" + nl +
       "WARNING: Group deletion only works for old ZK-based consumer groups, and one has to use it carefully to only delete groups that are not active."
-    val NewConsumerDoc = "Use new consumer. This is the default."
+    val NewConsumerDoc = "Use new consumer. This option requires that the 'bootstrap-server' option is used."
     val TimeoutMsDoc = "The timeout that can be set for some use cases. For example, it can be used when describing the group " +
       "to specify the maximum amount of time in milliseconds to wait before the group stabilizes (when the group is just created, " +
       "or is going through some changes)."