Document the new auto_alias tag in the serive container in 2.7

[fabpot]

From symfony/symfony#12526

Docs is mostly here symfony/symfony#11460 (comment)

[wouterj]

This at least has to be documented in reference/dic_tags. Besides that, it may even deserve a cookbook recipe

[javiereguiluz]

For reference purposes, here it is the (brief) blog post explaining this new feature: http://symfony.com/blog/new-in-symfony-2-7-dependency-injection-improvements#added-an-auto-alias-compiler-pass