add missing crosslinks between authorization and voters

I experienced a lack of internal crosslinks. Especially when I was faced with the page: http://symfony.com/doc/current/components/security/authorization.html#access-decision-manager

I was not able to find the configuration snippet as fast as it could be. I came from a search on google to the page and as a user I should be able quickly to jump to any place with related content from this point.

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On the page http://symfony.com/doc/current/cookbook/security/voters.html#changing-the-access-decision-strategy the description about the different strategies is not good as on the other one as well. Thats another reason why I added a backlink.

PS: Sorry, Iam not sure if the links correct in this way I added them, I tried. I can also change the box 'note' to a 'tip', if that makes more sense.