Sample code for hook_entity_field_storage_info() is too specific

Problem/Motivation

API page: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Entity%21...

The sample code for this hook is taken from field module's implementation, which dynamically creates field storage for all config fields.

It's a very complex example, and not that helpful, as it doesn't show a field storage object actually being defined in code.

The code in this test implementation would be much better: https://api.drupal.org/api/drupal/core%21modules%21system%21tests%21modu...

Steps to reproduce

Proposed resolution

Remaining tasks

Postponed on 📌 Add the FieldStorageDefinition class to define field storage definitions in hook_entity_field_storage_info() Needs review
When no longer postponed, update the instructions for a first time contributor and add back novice tag.

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet