Accessibility: add missing labels for Plugins and Themes checkboxes in the Updates screen.

Also, adds translator comments and improves readability.

Props SergeyBiryukov, afercia, oaron for the initial patch.
Fixes #34774.
Built from https://develop.svn.wordpress.org/trunk@35952


git-svn-id: https://core.svn.wordpress.org/trunk@35916 1a063a9b-81f0-0310-95a4-ce76da25c4cd