Document why we don't merge carts

[bojanz]

http://ryanszrama.com/blog/01-31-2015/why-not-combine-shopping-carts-user-login

This blog post needs to be converted into docs, along with some basic info on how people can do it anyway (respond to a cart event, etc).

[krcha]

This should be documented here:
https://docs.drupalcommerce.org/commerce2/developer-guide/cart/customize-add-to-cart-form

[bojanz]

There is a code sample at https://www.drupal.org/project/commerce/issues/2911567

[star-szr]

Archived version since the original post seems to be gone: https://web.archive.org/web/20201203161919/http://ryanszrama.com/blog/01-31-2015/why-not-combine-shopping-carts-user-login