Document usage of backoff policy in BulkProcessor

With this commit we update the documentation to explain the
new backoff feature in BulkProcessor.

Relates to #14620.

docs/java-api/docs/bulk.asciidoc

@@ -47,6 +47,7 @@ To use it, first create a `BulkProcessor` instance:
 
 [source,java]
 --------------------------------------------------
+import org.elasticsearch.action.bulk.BackoffPolicy;
 import org.elasticsearch.action.bulk.BulkProcessor;
 import org.elasticsearch.common.unit.ByteSizeUnit;
 import org.elasticsearch.common.unit.ByteSizeValue;
@@ -73,6 +74,8 @@ BulkProcessor bulkProcessor = BulkProcessor.builder(
         .setBulkSize(new ByteSizeValue(1, ByteSizeUnit.GB)) <6>
         .setFlushInterval(TimeValue.timeValueSeconds(5)) <7>
         .setConcurrentRequests(1) <8>
+        .setBackoffPolicy(
+            BackoffPolicy.exponentialBackoff(TimeValue.timeValueMillis(100), 3)) <9>
         .build();
 --------------------------------------------------
 <1> Add your elasticsearch client
@@ -86,6 +89,10 @@ BulkProcessor bulkProcessor = BulkProcessor.builder(
 <7> We want to flush the bulk every 5 seconds whatever the number of requests
 <8> Set the number of concurrent requests. A value of 0 means that only a single request will  be allowed to be
     executed. A value of 1 means 1 concurrent request is allowed to be executed while accumulating new bulk requests.
+<9> Set a custom backoff policy which will initially wait for 100ms, increase exponentially and retries up to three
+    times. A retry is attempted whenever one or more bulk item requests have failed with an `EsRejectedExecutionException`
+    which indicates that there were too little compute resources available for processing the request. To disable backoff,
+    pass `BackoffPolicy.noBackoff()`.
 
 Then you can simply add your requests to the `BulkProcessor`:
 
@@ -101,6 +108,7 @@ By default, `BulkProcessor`:
 * sets bulkSize to `5mb`
 * does not set flushInterval
 * sets concurrentRequests to 1
+* sets backoffPolicy to an exponential backoff with 8 retries and a start delay of 50ms. The total wait time is roughly 5.1 seconds.
 
 When all documents are loaded to the `BulkProcessor` it can be closed by using `awaitClose` or `close` methods: