Update permission docs

[Mathias-a]

@eilifhl tried to use it to implement permissions, and did not clearly understand the following from the docs:

• What the hell are all these "SAMFUNDET_VIEW_RECRUITMENT" etc. permissions
• Why do the get_queryset functions often have "SAMFUNDET_VIEW_" mentioned in them?
• Explicit explanation on how permission classes protect views
• Explain how a user is granted access, ie. how does the backend know that a user has access? (Someone defined it in the backend)
• Explain roles maybe?
• Explain when it's useful to use role system permissions, and when to use model permissions (TL;DR: use roles when instances matter, eg. recruitment where Isift org should not access Samf recruitment, models are more general)