Document how to workaround the fact that the TBB 'Print preview' is blank

Reported by user, confirmed by me and anonym:
Steps to reproduce:

- Open Browser
- Select option ‘print’ from menu

Error:
The print appears blank

What should happen:
Preview of the page

Happens in Tails 1.5.1. Does not happen in Tails 1.4.1 nor in Debian/Jessie with last TBB. Not

The page is actually printed if you click the print option on the preview.
Maybe AppArmor?

Feature Branch: doc/10141-print-preview

Original created by @emmapeel on 10141 (Redmine)

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