Improve how media embed default alt text is conveyed to assistive technology

Problem/Motivation

Follow-up to #3246365: [drupalMedia] Show the Image Media's default alt text that is being overridden.

Quote from @bnjmnm on #3246365-26: [drupalMedia] Show the Image Media's default alt text that is being overridden:

The fact that AT does not know the existing alt text value is a concern. It's not straightforward as it's not the value of the field... but it is changing a value, and typically the expectation would be that value is conveyed in the input. Visually, this is fairly easy to figure out based on the clear label above the input.

There's a similar issue in CKEditor 4 as the value-to-be-overridden is placeholder text #3087535: In WYSIWYG embed modal for existing images, the alt text field should have a default value instead of placeholder text, so doing this wouldn't be a regression that prevented this from landing.

I'd like a followup created to investigate making the default value more apparent to AT, perhaps by associating the "default alternative text" label with the input as a description (with additional visually hidden text if needed).

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet