[2.6] Document extensibility of ExpressionLanguage

[xabbuh]

Symfony got some new features based on the ExpressionLanguage with 2.6 (see symfony/symfony#12006).

[wouterj]

To be precise, things that have to be done:

The main change is in the ExpressionLanguage, where the ExpressionFunction class and ExpressionFunctionProviderInterface are added to easily add custom functions to the expression language.
This change has to be documented in components/expression_language/

This has been implemented for the Security and Routing using the routing.expression_language_provider and security.expression_language_provider tags. This needs to be documented in reference/dic_tags