Update FAQ
Update FAQ

This is a fall-out of updating the documentation of 1.6.

However, all the updates here are not 1.6-specific, so I want to merge
them into master directly. (Which simplifies things as there are some
FAQ updates in master already that are not in the next-release branch
yet.)

du2016/prometheus-docs,du2016/prometheus-docs,ddfreyne/prometheus-docs,prometheus/docs,chosenken/docs,chosenken/docs,du2016/prometheus-docs,ddfreyne/prometheus-docs,chosenken/docs,chosenken/docs,prometheus/docs,ddfreyne/prometheus-docs,prometheus/docs,ddfreyne/prometheus-docs