[Content]: Promotable explanation for Products/variants

[nfourtythree]

Product

Commerce

Description

Currently, the docs say that new products are promotable by default. In Commerce 5 the promotable property has been moved to the variant (purchasable) and by default they are not promotable. This is a decision to try to prevent people from accidentally selling things at a discount.

The docs need to be updated to reflect this behaviour.

Related: craftcms/commerce#3571