Final additions: Expression language framework docs

Hi guys!

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#8913)
| Applies to    | 2.4+
| Fixed tickets | #3022 

This finishes the framework documentation for the expression additions. I want to merge this ASAP, so if you can proofread this, I would very much appreciate it! I'm sure there are at least a few typos here and there, but hopefully everything is at least complete :).

Thanks!