MINOR: Improve docs for client group configs (#19605)

Improve java docs for session and HB interval client configs & fix
max.poll.interval description

Reviewers: David Jacot <djacot@confluent.io>

clients/src/main/java/org/apache/kafka/clients/CommonClientConfigs.java

@@ -192,7 +192,8 @@ public class CommonClientConfigs {
                                                           + "is considered failed and the group will rebalance in order to reassign the partitions to another member. "
                                                           + "For consumers using a non-null <code>group.instance.id</code> which reach this timeout, partitions will not be immediately reassigned. "
                                                           + "Instead, the consumer will stop sending heartbeats and partitions will be reassigned "
-                                                          + "after expiration of <code>session.timeout.ms</code>. This mirrors the behavior of a static consumer which has shutdown.";
+                                                          + "after expiration of the session timeout (defined by the client config <code>session.timeout.ms</code> if using the Classic rebalance protocol, or by the broker config <code>group.consumer.session.timeout.ms</code> if using the Consumer protocol). "
+                                                          + "This mirrors the behavior of a static consumer which has shutdown.";
 
     public static final String REBALANCE_TIMEOUT_MS_CONFIG = "rebalance.timeout.ms";
     public static final String REBALANCE_TIMEOUT_MS_DOC = "The maximum allowed time for each worker to join the group "
@@ -206,15 +207,18 @@ public class CommonClientConfigs {
                                                         + "to the broker. If no heartbeats are received by the broker before the expiration of this session timeout, "
                                                         + "then the broker will remove this client from the group and initiate a rebalance. Note that the value "
                                                         + "must be in the allowable range as configured in the broker configuration by <code>group.min.session.timeout.ms</code> "
-                                                        + "and <code>group.max.session.timeout.ms</code>. Note that this configuration is not supported when <code>group.protocol</code> "
+                                                        + "and <code>group.max.session.timeout.ms</code>. Note that this client configuration is not supported when <code>group.protocol</code> "
-                                                        + "is set to \"consumer\".";
+                                                        + "is set to \"consumer\". In that case, session timeout is controlled by the broker config <code>group.consumer.session.timeout.ms<code>.";
 
     public static final String HEARTBEAT_INTERVAL_MS_CONFIG = "heartbeat.interval.ms";
     public static final String HEARTBEAT_INTERVAL_MS_DOC = "The expected time between heartbeats to the consumer "
                                                            + "coordinator when using Kafka's group management facilities. Heartbeats are used to ensure that the "
                                                            + "consumer's session stays active and to facilitate rebalancing when new consumers join or leave the group. "
-                                                           + "The value must be set lower than <code>session.timeout.ms</code>, but typically should be set no higher "
+                                                           + "This config is only supported if <code>group.protocol</code> is set to \"classic\". In that case, "
-                                                           + "than 1/3 of that value. It can be adjusted even lower to control the expected time for normal rebalances.";
+                                                           + "the value must be set lower than <code>session.timeout.ms</code>, but typically should be set no higher "
+                                                           + "than 1/3 of that value. It can be adjusted even lower to control the expected time for normal rebalances."
+                                                           + "If <code>group.protocol</code> is set to \"consumer\", this config is not supported, as "
+                                                           + "the heartbeat interval is controlled by the broker with <code>group.consumer.heartbeat.interval.ms<code>.";
 
     public static final String DEFAULT_API_TIMEOUT_MS_CONFIG = "default.api.timeout.ms";
     public static final String DEFAULT_API_TIMEOUT_MS_DOC = "Specifies the timeout (in milliseconds) for client APIs. " +

clients/src/main/java/org/apache/kafka/clients/consumer/internals/AbstractHeartbeatRequestManager.java

@@ -63,8 +63,9 @@ public abstract class AbstractHeartbeatRequestManager<R extends AbstractResponse
     protected final Logger logger;
 
     /**
-     * Time that the group coordinator will wait on member to revoke its partitions. This is provided by the group
+     * Max time allowed between invocations of poll, defined in the {@link ConsumerConfig#MAX_POLL_INTERVAL_MS_CONFIG} config.
-     * coordinator in the heartbeat
+     * This is sent to the coordinator in the first heartbeat to join a group, to be used as rebalance timeout.
+     * Also, the consumer will proactively rejoin the group on a call to poll if this time has expired.
      */
     protected final int maxPollIntervalMs;