[DOCS] allow_duplicates option for append processor (#62336) (#63576)

Author: danhermann

docs/reference/ingest/processors/append.asciidoc

@@ -17,6 +17,8 @@ Accepts a single value or an array of values.
 | Name      | Required  | Default  | Description
 | `field`  | yes       | -        | The field to be appended to. Supports <<accessing-template-fields,template snippets>>.
 | `value`  | yes       | -        | The value to be appended. Supports <<accessing-template-fields,template snippets>>.
+| `allow_duplicates` | no | true  | If `false`, the processor does not append
+values already present in the field.
 include::common-options.asciidoc[]
 |======