Document that views bulk forms no longer have a default value (and why)

Problem/Motivation

Very early in the 10.1 cycle #3307509: No empty option for views bulk form removed the default value from all core views bulk forms. This is not in the release notes and there is no change record, so I had to do some digging to find when this happened.

Quoting xjm:

IMO this is a major usability bug, so promoting and retitling accordingly

This broke some of our functional tests when we updated to 10.1. Those test are easy to fix once you realise what is going on, but at first we asked ourselves wether to just accept this new situation or maybe bring back the old default values with some configuration or custom code. As far as I could see there is no way to configure this, so it is either accepting this change or adding custom code.

- Do we want a change record for this?
- Is this a new recommended UI pattern that should be documented as such so that contrib can make sure to follow this pattern at all times?
(Possibly at https://www.drupal.org/docs/develop/user-interface-standards)

Steps to reproduce

Proposed resolution

Add a change record for #3307509: No empty option for views bulk form ?
Add this to https://www.drupal.org/docs/develop/user-interface-standards or some other forms UI patterns section?

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet