document vcr_turn_off/et al. env vars better

[sckott]

via #37

https://github.com/ropensci/vcr/issues/37#issuecomment-566212328 and my comment after

[maelle]

I was thinking that for these variables and other lightswitch options a decision tree would be good (a diagram, or in text form).

You want to turn off vcr...

• in a single test, blabla

• in all tests, blabla

well with more decision criteria.

[sckott]

good idea!