Docs: Add a missing DocBlock summary and `$action` parameter description for...

Docs: Add a missing DocBlock summary and `$action` parameter description for `wp_ajax_delete_page()`.

Props abhijitrakas.
Fixes #44206.

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git-svn-id: https://core.svn.wordpress.org/trunk@44165 1a063a9b-81f0-0310-95a4-ce76da25c4cd