Add description when fetch document item

The endpoint GET /api/documents/{id} return a list of items but the item
haven't the description. With this update will return the description of
a item.

Signed-off-by: Vitor Mattos <vitor@php.rio>

app/Http/Resources/Document/DocumentItem.php

@@ -22,6 +22,7 @@ public function toArray($request)
             'document_id' => $this->document_id,
             'item_id' => $this->item_id,
             'name' => $this->name,
+            'description' => $this->description,
             'price' => $this->price,
             'price_formatted' => money($this->price, $this->document->currency_code, true)->format(),
             'total' => $this->total,