[doc][client] Add timestamp format description for seek method of the…

… Consumer and Reader (apache#16648)

### Motivation

There are many timestamp formats, we need to specify the required timestamp format for the seek method of the Consumer and Reader.

### Modifications
* Add a description requiring the timestamp format should be Unix Time in milliseconds for the seek method of the Consumer and Reader.

pulsar-client-api/src/main/java/org/apache/pulsar/client/api/Consumer.java

@@ -708,6 +708,7 @@ CompletableFuture<Void> reconsumeLaterCumulativeAsync(Message<?> message,
      *
      * @param timestamp
      *            the message publish time where to reposition the subscription
+     *            The timestamp format should be Unix time in milliseconds.
      */
     void seek(long timestamp) throws PulsarClientException;
 
@@ -765,6 +766,7 @@ CompletableFuture<Void> reconsumeLaterCumulativeAsync(Message<?> message,
      *
      * @param timestamp
      *            the message publish time where to reposition the subscription
+     *            The timestamp format should be Unix time in milliseconds.
      * @return a future to track the completion of the seek operation
      */
     CompletableFuture<Void> seekAsync(long timestamp);

pulsar-client-api/src/main/java/org/apache/pulsar/client/api/Reader.java

@@ -153,6 +153,7 @@ public interface Reader<T> extends Closeable {
      * the seek() on the individual partitions.
      *
      * @param timestamp the message publish time where to reposition the reader
+     *                  The timestamp format should be Unix time in milliseconds.
      */
     void seek(long timestamp) throws PulsarClientException;
 
@@ -212,6 +213,7 @@ public interface Reader<T> extends Closeable {
      *
      * @param timestamp
      *            the message publish time where to position the reader
+     *            The timestamp format should be Unix time in milliseconds.
      * @return a future to track the completion of the seek operation
      */
     CompletableFuture<Void> seekAsync(long timestamp);