docs: clarify Mercure URLs (#916)

* clarify mercure URLs

* Update docs/mercure.md

Co-authored-by: David Buchmann <david@liip.ch>

* Update mercure.md

---------

Co-authored-by: Kévin Dunglas <kevin@dunglas.fr>

Author: dbu

docs/mercure.md

@@ -1,12 +1,15 @@
 # Real-time
 
 FrankenPHP comes with a built-in [Mercure](https://mercure.rocks) hub!
-Mercure allows to push events in real-time to all the connected devices: they will receive a JavaScript event instantly.
+Mercure allows you to push real-time events to all the connected devices: they will receive a JavaScript event instantly.
 
-No JS library or SDK required!
+No JS library or SDK is required!
 
 ![Mercure](mercure-hub.png)
 
 To enable the Mercure hub, update the `Caddyfile` as described [on Mercure's site](https://mercure.rocks/docs/hub/config).
 
-To push Mercure updates from your code, we recommend the [Symfony Mercure Component](https://symfony.com/components/Mercure) (you don't need the Symfony full stack framework to use it).
+The path of the Mercure hub is `/.well-known/mercure`. 
+When running FrankenPHP inside Docker, the full send URL would look like `http://php/.well-known/mercure` (with `php` being the container's name running FrankenPHP).
+
+To push Mercure updates from your code, we recommend the [Symfony Mercure Component](https://symfony.com/components/Mercure) (you don't need the Symfony full-stack framework to use it).