Press This and admin tag meta-box: use aria-describedby to improve accessibility

[afercia]

The "new tag" input field has a description (in the .howto paragraph) that can be used as target for aria-describedby to improve feedback for screen reader users.

See proposed patch, for both Press This and the admin meta box.
Using "trunk" for version and "Press This" as component though this relates also to the admin.

[azaozz]

In 31808:

Press This: add aria-describedby to the new tag field.
Props afercia. Fixes #31546.

[azaozz]

@afercia: we cannot add the same code to the postbox. There can be more than one tags postboxes on the screen, will have to generate the HTML IDs by using the taxonomy name, etc.

[afercia]

Replying to azaozz:

@afercia: we cannot add the same code to the postbox.

@azaozz: got it, thanks very much