Docs: Improve documentation for functions in `wp-admin/install-helper.php` per...

Docs: Improve documentation for functions in `wp-admin/install-helper.php` per the documentation standards.

Synchronize descriptions with the functions' counterparts from `wp-admin/includes/upgrade.php`.

See #49572.
Built from https://develop.svn.wordpress.org/trunk@47786


git-svn-id: https://core.svn.wordpress.org/trunk@47562 1a063a9b-81f0-0310-95a4-ce76da25c4cd