Update api_products.class.php

fixes #14309 
Throw 400 and add hint to user about using the right endpoint.

htdocs/product/class/api_products.class.php

@@ -286,6 +286,9 @@ class Products extends DolibarrApi
 
         foreach ($request_data as $field => $value) {
             if ($field == 'id') { continue;
+            }
+			if ($field == 'stock_reel') {
+				throw new RestException(400, 'Stock reel cannot be updated here. Use the /stockmovements endpoint instead');
             }
             $this->product->$field = $value;
         }