Update docs for bulk helper

Adds a section detailing how to modify documents before sending to the
bulk API, using the improvement introduced in
#1732

Author: JoshMock

docs/helpers.asciidoc

@@ -326,6 +326,33 @@ const result = await client.helpers.bulk({
 console.log(result)
 ----
 
+[discrete]
+==== Modifying a document before operation
+
+~Added~ ~in~ ~`v8.8.2`~
+
+If you need to modify documents in your datasource before it is sent to Elasticsearch, you can return an array in the `onDocument` function rather than an operation object. The first item in the array must be the operation object, and the second item must be the document or partial document object as you'd like it to be sent to Elasticsearch.
+
+[source,js]
+----
+const { Client } = require('@elastic/elasticsearch')
+
+const client = new Client({
+  cloud: { id: '<cloud-id>' },
+  auth: { apiKey: 'base64EncodedKey' }
+})
+const result = await client.helpers.bulk({
+  datasource: [...]
+  onDocument (doc) {
+    return [
+      { index: { _index: 'my-index' } },
+      { ...doc, favorite_color: 'mauve' },
+    ]
+  }
+})
+
+console.log(result)
+----
 
 [discrete]
 [[multi-search-helper]]
@@ -574,4 +601,4 @@ const scrollSearch = client.helpers.scrollDocuments({
 for await (const doc of scrollSearch) {
   console.log(doc)
 }
-----
+----