Document the Java BulkProcessor

Closes #7638.

docs/java-api/bulk.asciidoc

@@ -36,3 +36,66 @@ if (bulkResponse.hasFailures()) {
     // process failures by iterating through each bulk response item
 }
 --------------------------------------------------
+
+[float]
+== Using Bulk Processor
+
+The `BulkProcessor` class offers a simple interface to flush bulk operations automatically based on the number or size
+of requests, or after a given period.
+
+To use it, first create a `BulkProcessor` instance:
+
+[source,java]
+--------------------------------------------------
+import org.elasticsearch.action.bulk.BulkProcessor;
+
+BulkProcessor bulkProcessor = BulkProcessor.builder(
+        client,  <1>
+        new BulkProcessor.Listener() {
+            @Override
+            public void beforeBulk(long executionId,
+                                   BulkRequest request) { ... } <2>
+
+            @Override
+            public void afterBulk(long executionId,
+                                  BulkRequest request,
+                                  BulkResponse response) { ... } <3>
+
+            @Override
+            public void afterBulk(long executionId,
+                                  BulkRequest request,
+                                  Throwable failure) { ... } <4>
+        })
+        .setBulkActions(10000) <5>
+        .setBulkSize(new ByteSizeValue(1, ByteSizeUnit.GB)) <6>
+        .setFlushInterval(TimeValue.timeValueSeconds(5)) <7>
+        .setConcurrentRequests(1) <8>
+        .build();
+--------------------------------------------------
+<1> Add your elasticsearch client
+<2> This method is called just before bulk is executed. You can for example see the numberOfActions with
+    `request.numberOfActions()`
+<3> This method is called after bulk execution. You can for example check if there was some failing requests
+    with `response.hasFailures()`
+<4> This method is called when the bulk failed and raised a `Throwable`
+<5> We want to execute the bulk every 10 000 requests
+<6> We want to flush the bulk every 1gb
+<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.
+
+Then you can simply add your requests to the `BulkProcessor`:
+
+[source,java]
+--------------------------------------------------
+bulkProcessor.add(new IndexRequest("twitter", "tweet", "1").source(/* your doc here */));
+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
+