Associate the terms form fields with their descriptions

[afercia]

The form fields to create and edit taxonomy terms in edit-tags.php and term.php do have descriptions. However, these descriptions are not semantically associated with the input fields.

By using the aria-describedby attribute, assistive technologies would be able to understand there's a programmatic association between the form fields and the descriptions. Screen readers, for example, would automatically read out the descriptions while tabbing through the form fields.

It's a small improvement but I'd say using aria-describedby in these cases should be a default practice.

See all the form fields (inlcuding the select element) in the screenshot below.

[audrasjb]

Moving for 6.1 consideration and adding good-first-bug workflow keyword as it looks like a great bugfix for new contributors :)

[afercia]

55651.diff​ is a first pass:

• Adds aria-describedby and id attributes to the form HTML.
• Adds an aria_describedby argument to wp_dropdown_categories(), to optionally print out an aria-describedby attribute.

Sorry for the good-first-bug keyword, I already had a patch ready :)

[joedolson]

In 53913:

Taxonomy: Associate field descriptions with fields.

Use aria-describedby to associate the field descriptions and instructions with the form fields when creating or editing taxonomy terms.

Props afercia.
Fixes #55651.