Indicate what entity(s) are blocking the use of the paired endpoint suffix

[westonruter]

Summary

Fixes #6339

Before	After

Testing

To test the functionality:

• Create a post with the slug amp.
• Create a page with the slug amp.
• Create a post tag with the slug amp.
• Create a category with the slug amp.
• Create a user with the username of amp.

Then:

• Register a post type with amp as the rewrite slug.
• Register a taxonomy with amp as the rewrite slug.
• Register an amp rewrite endpoint.

These can be achieved with this plugin code:

add_action(
	'init',
	function () {
		add_rewrite_endpoint( 'amp', E_ALL );
		register_post_type( 'amp-post',
			[
				'labels' => [
					'name'          => 'AMP Posts',
					'singular_name' => 'AMP Post',
				],
				'rewrite' => [
					'slug' => 'amp'
				],
			]
		);
		register_taxonomy( 'amp-tax', 'post', [
			'labels' => [
				'name'          => 'AMP Taxes',
				'singular_name' => 'AMP Tax',
			],
			'rewrite' => [
				'slug' => 'amp'
			],
		] );
	}
);

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[github-actions]

Plugin builds for fd93c82 are ready 🛎️!

• Download development build
• Download production build