Accessibility: Improvements for the "Send Trackbacks" postbox.

Adds an `aria-describedby` attribute targeting the input field description and styles the description as a... description.

Fixes #35123.
Built from https://develop.svn.wordpress.org/trunk@35983


git-svn-id: https://core.svn.wordpress.org/trunk@35948 1a063a9b-81f0-0310-95a4-ce76da25c4cd