UPDATE_ACTION_DELETE should document that it works by updating the alias entity

Problem/Motivation

In the UI, the option for UPDATE_ACTION_DELETE talks of deleting the alias, which is correct for end-users:

> PathautoGeneratorInterface::UPDATE_ACTION_DELETE => $this->t('Create a new alias. Delete the old alias.'),

However, it's important for developers to know that this option works by *updating* the alias value of the existing path_alias entity.

This should be documented somewhere, probably on the UPDATE_ACTION_DELETE constant.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes