Continue ibexa_product_specification

Author: adriendupuis

docs/content_management/field_types/field_type_reference/productspecificationfield.md

@@ -1,11 +1,20 @@
 ---
 edition: headless
+month_change: true
 ---
 
 # Product specification field type
 
+This field represents and handles [product attributes](products.md#product-attributes) and [VAT](prices.md#vat).
+Consider it as internal to the [PIM](pim.md). 
+
 | Name                   | Internal name                 | Expected input |
 |------------------------|-------------------------------|----------------|
 | `ProductSpecification` | `ibexa_product_specification` | mixed          |
 
-TODO
+!!! caution
+
+    The presence of a specification (`ibexa_product_specification`) field is what distinct product type from content type.
+    Don't remove this field from a product type (or it becomes a unreachable hidden content type).
+    Don't add such field to a content type (or it becomes an uneditable unusable product type).
+    You can't have more than one `ibexa_product_specification`, the laters won't be editable.