MINOR: Update max.connections.per.ip.overrides config docs (apache#4819)

Add a validation check to make sure max.connections.per.ip.overrides is configured when max.connections.per.ip is set zero. Also clean up the config description.

core/src/main/scala/kafka/server/KafkaConfig.scala

@@ -517,8 +517,9 @@ object KafkaConfig {
   val SocketSendBufferBytesDoc = "The SO_SNDBUF buffer of the socket sever sockets. If the value is -1, the OS default will be used."
   val SocketReceiveBufferBytesDoc = "The SO_RCVBUF buffer of the socket sever sockets. If the value is -1, the OS default will be used."
   val SocketRequestMaxBytesDoc = "The maximum number of bytes in a socket request"
-  val MaxConnectionsPerIpDoc = "The maximum number of connections we allow from each ip address"
-  val MaxConnectionsPerIpOverridesDoc = "Per-ip or hostname overrides to the default maximum number of connections"
+  val MaxConnectionsPerIpDoc = "The maximum number of connections we allow from each ip address. This can be set to 0 if there are overrides " +
+    "configured using " + MaxConnectionsPerIpOverridesProp + " property"
+  val MaxConnectionsPerIpOverridesDoc = "A comma-separated list of per-ip or hostname overrides to the default maximum number of connections. An example value is \"hostName:100,127.0.0.1:200\""
   val ConnectionsMaxIdleMsDoc = "Idle connections timeout: the server socket processor threads close the connections that idle more than this"
   /************* Rack Configuration **************/
   val RackDoc = "Rack of the broker. This will be used in rack aware replication assignment for fault tolerance. Examples: `RACK1`, `us-east-1d`"
@@ -1356,5 +1357,9 @@ class KafkaConfig(val props: java.util.Map[_, _], doLog: Boolean, dynamicConfigO
       s"${KafkaConfig.SaslMechanismInterBrokerProtocolProp} must be included in ${KafkaConfig.SaslEnabledMechanismsProp} when SASL is used for inter-broker communication")
     require(queuedMaxBytes <= 0 || queuedMaxBytes >= socketRequestMaxBytes,
       s"${KafkaConfig.QueuedMaxBytesProp} must be larger or equal to ${KafkaConfig.SocketRequestMaxBytesProp}")
+
+    if (maxConnectionsPerIp == 0)
+      require(!maxConnectionsPerIpOverrides.isEmpty, s"${KafkaConfig.MaxConnectionsPerIpProp} can be set to zero only if" +
+        s" ${KafkaConfig.MaxConnectionsPerIpOverridesProp} property is set.")
   }
 }

core/src/test/scala/unit/kafka/server/KafkaConfigTest.scala

@@ -780,6 +780,15 @@ class KafkaConfigTest {
     assertFalse(isValidKafkaConfig(props))
   }
 
+  @Test
+  def testMaxConnectionsPerIpProp() {
+    val props = TestUtils.createBrokerConfig(0, TestUtils.MockZkConnect, port = 8181)
+    props.put(KafkaConfig.MaxConnectionsPerIpProp, "0")
+    assertFalse(isValidKafkaConfig(props))
+    props.put(KafkaConfig.MaxConnectionsPerIpOverridesProp, "127.0.0.1:100")
+    assertTrue(isValidKafkaConfig(props))
+  }
+
   private def assertPropertyInvalid(validRequiredProps: => Properties, name: String, values: Any*) {
     values.foreach((value) => {
       val props = validRequiredProps