minor #14404 Made all messenger transport look the same (Nyholm)

This PR was squashed before being merged into the 5.x branch.

Discussion
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Made all messenger transport look the same

This PR is fixing the Transport Configuration in the Messenger component. https://symfony.com/doc/5.2/messenger.html#transport-configuration

I added the following order:

```
Foo Transport
~~~~~~~~~~

One line introduction of Foo. Install it by running:

.. code-block:: terminal

    $ composer require symfony/foo-messenger

The Foo transport DSN may looks like this:

.. code-block:: env

    # .env
    MESSENGER_TRANSPORT_DSN=foo://default

[Any other info about the transport]

The transport has a number of options:

[Table with options]

[Other notes and option related stuff.]
```

Commits
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a93da4e Made all messenger transport look the same

Author: javiereguiluz