Decide what to do with thumbnail_alt_metadata_attribute and related documentation

Problem/Motivation

This is a followup from 📌 Convert MediaSource plugin discovery to attributes Active based on this comment by alexpott.

The documentation for \Drupal\media\Attribute\MediaSource::__construct(thumbnail_alt_metadata_attribute) is incorrect. It is used in \Drupal\media\Entity\Media::updateThumbnail() but if this is not set we don't set the thumbnail alt. This is my summary of that comment.

This issues is to discuss and fix it.

Steps to reproduce

Proposed resolution

TBD

Remaining tasks

1) Decide what to do with thumbnail_alt_metadata_attribute and related documentation

User interface changes

API changes

Data model changes

Release notes snippet