Tried to clarify #3255
[#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
Rewording security sections to be even more clear that secure_channel and ip don't actually *restrict* access
Added Caching article
[Cookbook] Fixes a link to the Request class
Added doctrine note
Merge pull request #3261 from zczapran/2.4

[Cookbook] Fixes a link to the Request class
Merge pull request #3260 from WouterJ/component_expression_caching

[ExpressionLanguage] Documented Caching Strategies
[#3260] Minor tweaks when proofreading
Merge pull request #3256 from WouterJ/clarify_checking

[#3255] Tried to clarify how to check a box
changed version from 2.3.0 --> 2.3.7

just for convenience (i.e. people not reading the text carefully but copying the example and starting with an "old" Symfony version)
[#3252] Simplifying version constraint by using a wildcard
fixes #3267

Make sure we use camelCase for action arguments