Docs: Synchronize `@return` descriptions for `::handle_row_actions()` methods in list tables.

Make sure `WP_Comments_List_Table::handle_row_actions()` and `WP_MS_Sites_List_Table::handle_row_actions()` return a string, for consistency with other classes.

See #49170, #48303.
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git-svn-id: https://core.svn.wordpress.org/trunk@46859 1a063a9b-81f0-0310-95a4-ce76da25c4cd