There is no such concept as a foreign key constraint in RavenDB. Really, there is no foreign key. All you have in your document is a string that can be used to go fetch another document. We call that a relationship, but it is a loose term. There is nothing going on under the hood to link those documents together and prevent you from doing that delete.
In the scenario you described, you would still be able to load the product. But you would get a null if you tried to load the category using that product's category id.
You have to maintain these relationships yourself. The only helper available is the Cascade Delete Bundle - but you still must wire up each entity for cascading. It doesn't do it automatically like it would with a sql foreign key based cascade delete.