Update libbeat/processors/actions/docs/decode_json_fields.asciidoc

libbeat/processors/actions/docs/decode_json_fields.asciidoc

@@ -38,7 +38,8 @@ default value is `false`.
 `expand_keys`:: (Optional) A Boolean value that specifies whether keys in the decoded JSON
 should be recursively de-dotted and expanded into a hierarchical object structure.
 For example, `{"a.b.c": 123}` would be expanded into `{"a":{"b":{"c":123}}}`.
-`add_error_key`:: (Optional) If set to `true`, the `error` field is a part of the event with the error message. If set to `false`, there will not be any error in the event field. This error occurs while decoding JSON keys. The default value is `false`.
+`add_error_key`:: (Optional) If set to `true` and an error occurs while decoding JSON keys,
+the `error` field will become a part of the event with the error message. If set to `false`, there will not be any error in the event's field. The default value is `false`.
 `document_id`:: (Optional) JSON key that's used as the document ID. If configured,
 the field will be removed from the original JSON document and stored in
 `@metadata._id`