When nav menu location slugs are integers, some hard to debug results can occur. `register_nav_menus()` utilizes `array_merge()` which renumbers numeric indexes, starting from 0. Because of this, numeric menu locations will almost always be changed.
This change introduces a `_doing_it_wrong()` notice to inform developers that nav menu locations should always be strings.
Props audrasjb, desrosj, welcher.
Fixes#45361.
git-svn-id: https://develop.svn.wordpress.org/trunk@46102 602fd350-edb4-49c9-b593-d223f7449a82
50a0d6c7a9