Improve documentation around enable-if-true/disable-if-false

[jamezpolley]

At this point, I have a pretty solid grasp of the difference and how they intereact, I think. But it's taken me quite a while to get there.

I would like to beef up the docs around these conditions:

• Highlight common errors
  • create a voucher with an enable-if-true on a dinner ticket to let someone who wouldn't ordinarily be eligible buy a ticket -> now nobody else can buy a ticket)
  • Create one voucher with disable-if-false - all works fine, only people with that voucher can buy the product. Create a second disable-if-false voucher for the same product, and now nobody can buy the product unless they have both vouchers..