Improve docs for admin route definition

[latenzio]

Short description of what this feature will allow to do:
Make easy admin dashboard route respect locale detection. Enable redirect from /admin to /admin/[DETECTED_LOCALE].
Please enhancement examples in docs.

Example of how to use this feature

#[Route('/admin/{_locale}', name: 'admin', defaults: ['_locale' => null])]

framework:
    set_locale_from_accept_language: true
    set_content_language_from_locale: true

[andersonamuller]

Could you please help test if this feature still works with the below patch?

#6954