Previously the label just said "Posts", "Pages", or "Comments". This was bad in terms of accessibility and internationalization because of missing context.
This change adds a default label "Number of items per page:" to `WP_Screen->render_per_page_options()` and removes all the existing one-word labels.
props afercia.
fixes#31349, #15576.
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git-svn-id: http://core.svn.wordpress.org/trunk@31677 1a063a9b-81f0-0310-95a4-ce76da25c4cd