How Eloquent determines the foreign key of the relationship?

The documentation says:

Eloquent determines the foreign key of the relationship based on the model name.

but in the HasRelationship class in the belongsTo method, I can see this comment

If no foreign key was supplied, we can use a backtrace to guess the proper foreign key name by using the name of the relationship function, which when combined with an "_id" should conventionally match the columns.

So.. the documentation is wrong?

I've tested it and is true..so how it works exactly? (I already know how to set it manually)

Thanks.