Update access_control.rst

[kockodev]

Extend documentation with IP netmask

[javiereguiluz]

@rapemer just asking. Do this work in Symfony 2.7 too? If yes, then we'll merge this on 2.7 branch. Thanks.

[kockodev]

@javiereguiluz since this commit symfony/symfony@d9ac571#diff-23b6cd28961e2f301e71e2632ba728ab is available in IpUtils ... Since this PR symfony/symfony#7249 is documentation fully supported. I think branch 2.7 is fully supported.

[javiereguiluz]

Thanks! So, it's available since Symfony 2.3 and we'll then merge it on 2.7 branch. But there's no need for you to rebase this. We'll do that when merging. Thanks!

[javiereguiluz]

@rapemer this was a very nice first contribution to Symfony Docs. Thank you!