Explain how to use bulk processor in a test context

When using a bulk processor in test, you might write something like:

```java
BulkProcessor bulkProcessor = BulkProcessor.builder(client, new BulkProcessor.Listener() {
    @Override public void beforeBulk(long executionId, BulkRequest request) {}
    @Override public void afterBulk(long executionId, BulkRequest request, BulkResponse response) {}
    @Override public void afterBulk(long executionId, BulkRequest request, Throwable failure) {}
})
        .setBulkActions(10000)
        .setFlushInterval(TimeValue.timeValueSeconds(10))
        .build();

for (int i = 0; i < 10000; i++) {
    bulkProcessor.add(new IndexRequest("foo", "bar", "doc_" + i)
            .source(jsonBuilder().startObject().field("foo", "bar").endObject()
    ));
}

bulkProcessor.flush();
client.admin().indices().prepareRefresh("foo").get();
SearchResponse response = client.prepareSearch("foo").get();
// response does not contain any hit
```

The problem is that by default bulkProcessor defines the number of concurrent requests to 1 which is using behind the scene an Async BulkRequestHandler.
When you call `flush()` in a test, you expect it to flush all the content of the bulk so you can search for your docs.
But because of the async handling, there is a great chance that none of the documents has been indexed yet when you call the `refresh` method.

We should advice in our Java guide to explicitly set concurrent requests to `0` so users will use behind the scene the Sync BulkRequestHandler.

```java
BulkProcessor bulkProcessor = BulkProcessor.builder(client, new BulkProcessor.Listener() {
    @Override public void beforeBulk(long executionId, BulkRequest request) {}
    @Override public void afterBulk(long executionId, BulkRequest request, BulkResponse response) {}
    @Override public void afterBulk(long executionId, BulkRequest request, Throwable failure) {}
})
        .setBulkActions(5000)
        .setFlushInterval(TimeValue.timeValueSeconds(10))
        .setConcurrentRequests(0)
        .build();
```

Closes #22158.

docs/java-api/docs/bulk.asciidoc

@@ -87,13 +87,24 @@ BulkProcessor bulkProcessor = BulkProcessor.builder(
 <5> We want to execute the bulk every 10 000 requests
 <6> We want to flush the bulk every 5mb
 <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
+<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()`.
 
+By default, `BulkProcessor`:
+
+* sets bulkActions to `1000`
+* sets bulkSize to `5mb`
+* does not set flushInterval
+* sets concurrentRequests to 1, which means an asynchronous execution of the flush operation.
+* sets backoffPolicy to an exponential backoff with 8 retries and a start delay of 50ms. The total wait time is roughly 5.1 seconds.
+
+[[java-docs-bulk-processor-requests]]
+==== Add requests
+
 Then you can simply add your requests to the `BulkProcessor`:
 
 [source,java]
@@ -102,13 +113,8 @@ bulkProcessor.add(new IndexRequest("twitter", "tweet", "1").source(/* your doc h
 bulkProcessor.add(new DeleteRequest("twitter", "tweet", "2"));
 --------------------------------------------------
 
-By default, `BulkProcessor`:
-
-* sets bulkActions to `1000`
-* 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.
+[[java-docs-bulk-processor-close]]
+==== Closing the Bulk Processor
 
 When all documents are loaded to the `BulkProcessor` it can be closed by using `awaitClose` or `close` methods:
 
@@ -129,3 +135,33 @@ Both methods flush any remaining documents and disable all other scheduled flush
 all bulk requests to complete then returns `true`, if the specified waiting time elapses before all bulk requests complete,
 `false` is returned. The `close` method doesn't wait for any remaining bulk requests to complete and exits immediately.
 
+[[java-docs-bulk-processor-tests]]
+==== Using Bulk Processor in tests
+
+If you are running tests with elasticsearch and are using the `BulkProcessor` to populate your dataset
+you should better set the number of concurrent requests to `0` so the flush operation of the bulk will be executed
+in a synchronous manner:
+
+[source,java]
+--------------------------------------------------
+BulkProcessor bulkProcessor = BulkProcessor.builder(client, new BulkProcessor.Listener() { /* Listener methods */ })
+        .setBulkActions(10000)
+        .setConcurrentRequests(0)
+        .build();
+
+// Add your requests
+bulkProcessor.add(/* Your requests */);
+
+// Flush any remaining requests
+bulkProcessor.flush();
+
+// Or close the bulkProcessor if you don't need it anymore
+bulkProcessor.close();
+
+// Refresh your indices
+client.admin().indices().prepareRefresh().get();
+
+// Now you can start searching!
+client.prepareSearch().get();
+--------------------------------------------------
+