[#3022] Adding framework documentation for routing expressions
[#3022] Adding new Expression validation constraint
[#3022] Adding all of the security-related expression functionality docs
[#3022] Adding a quick cookbook entry to highlight where the expression component is used in the framework
[#3022][#3258] Tweaks thanks to @cordoval
[#3022][#3258] Enhancing example at @ggam's suggestion
[#3258] A bunch of fixes thanks to @WouterJ
Merge pull request #3258 from symfony/expression-language-framework

Final additions: Expression language framework docs