Document that this module also provides backreferences

I have installed Commerce Kickstart 2.x and have commerce_search_api enabled. I didn't need any of the features listed on the project's homepage, so I disabled it.

However, many of my custom rules that rely on going from a product to the node that references it were broken. Eg one rule that checks the cart for a product redirects users to commerce-product:field-product-node:0:url .

After re-enabling commerce_search_api, my rules became functional again.