Adds more information about ingest attachment properties extraction

This is coming from thsi thread on discuss: https://discuss.elastic.co/t/ingest-attachment-plugin-exception/69167/10

docs/plugins/ingest-attachment.asciidoc

@@ -52,7 +52,7 @@ The node must be stopped before removing the plugin.
 | `field`                | yes       | -                | The field to get the base64 encoded field from
 | `target_field`         | no        | attachment       | The field that will hold the attachment information
 | `indexed_chars`        | no        | 100000           | The number of chars being used for extraction to prevent huge fields. Use `-1` for no limit.
-| `properties`           | no        | all              | Properties to select to be stored. Can be `content`, `title`, `name`, `author`, `keywords`, `date`, `content_type`, `content_length`, `language`
+| `properties`           | no        | all properties   | Properties to select to be stored. Can be `content`, `title`, `name`, `author`, `keywords`, `date`, `content_type`, `content_length`, `language`
 | `ignore_missing`       | no        | `false`          | If `true` and `field` does not exist, the processor quietly exits without modifying the document
 |======
 
@@ -102,6 +102,25 @@ Returns this:
 --------------------------------------------------
 // TESTRESPONSE
 
+
+To specify only some fields to be extracted:
+
+[source,js]
+--------------------------------------------------
+PUT _ingest/pipeline/attachment
+{
+  "description" : "Extract attachment information",
+  "processors" : [
+    {
+      "attachment" : {
+        "field" : "data",
+        "properties": [ "content", "title" ]
+      }
+    }
+  ]
+}
+--------------------------------------------------
+
 NOTE: Extracting contents from binary data is a resource intensive operation and
       consumes a lot of resources. It is highly recommended to run pipelines
       using this processor in a dedicated ingest node.