Due to a confusion with `add_menu_page()`, which takes the `$icon_url` parameter, while `add_submenu_page()` does not, some plugins were passing in a string instead of integer as `$position`, causing backward compatibility issues.
A `_doing_it_wrong()` message is now added to alert developers of the wrong parameter type.
Props david.binda, desrosj, 123host, dennis_f, MattyRob.
Reviewed by desrosj.
Fixes#48249.
Built from https://develop.svn.wordpress.org/trunk@46570
git-svn-id: http://core.svn.wordpress.org/trunk@46367 1a063a9b-81f0-0310-95a4-ce76da25c4cd