Update doc wrt. burning a DVD using OSX

We currently tell Mac users to burn Tails ISO using Disk Utility, but this is no longer possible since OS X version El Capitan (june 2015) according to some recent users report and Apple website (https://support.apple.com/kb/PH22122).

OSX users only have to do right-click → Burn ISO to DVD on the ISO file.

So we should update our doc. And I’m volunteering!

Still as Macs don’t have DVD drive since 2013 I’m not putting it as elevated.

Related issues

Blocks #12432 (closed)

Original created by @mercedes508 on 12174 (Redmine)

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