Added mention about default values for migrations (#2349)

* Added mention about default values for migrations
* Applied review suggestion

Co-authored-by: Tomasz Dąbrowski <[email protected]>

---------

Co-authored-by: Tomasz Dąbrowski <[email protected]>

Author: mnocon

code_samples/data_migration/examples/create_product_type.yaml

@@ -18,6 +18,7 @@
         -   identifier: specification
             type: ibexa_product_specification
             required: true
+            translatable: false
             translations:
                 eng-GB:
                     name: Specification

docs/content_management/data_migration/importing_data.md

@@ -213,6 +213,11 @@ The following example shows how to create a content type with two Field definiti
 
 The required metadata keys are: `identifier`, `mainTranslation`, `contentTypeGroups` and `translations`.
 
+The default values of Field definition properties mirror the underlying PHP API, for example:
+
+- `translatable` defaults to `true`
+- `required` defaults to `false`
+
 ``` yaml
 [[= include_file('code_samples/data_migration/examples/create_blog_post_ct.yaml') =]]
 ```