[#3022] Adding service container expression language details

Hi guys!

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#8913)
| Applies to    | 2.4+
| Fixed tickets | #3022

Please read this over and let me know your thoughts! This is already such a big entry, so I wanted to (a) give all the information, (b) stay somewhat short and (c) not mess up any future examples (since we continue to use the `my_mailer` throughout this entry.

Thanks!