[Mercure] Add the reason for `--no-tls`

[seb-jean]

Hello :),
On the Mercure documentation page, in the "Running a Mercure Hub" section, there is the following sentence:

---

If you use the Symfony Local Web Server, you must start it with the --no-tls option.

symfony server:start --no-tls -d.

---

I find that the reason why we need to add --no-tls is missing.

[javiereguiluz]

Let's ping @dunglas to see if he can give us a hint about this. Thanks!

[dunglas]

It's to prevent mixed content and invalid TLS certificate issues.

[javiereguiluz]

Thanks Kévin! We'll add that to the docs.

[javiereguiluz]

Closed by #20945.