if zone creation becomes optional, document count caveat

[stavxyz]

If the variables referenced in an interpolation which is to be the value of count, then terraform must know the values of those variables/attributes ahead of time.

In the case tf can't know the values involved with computing the count, the user will see:

Error: Error refreshing state: 1 error(s) occurred:

* module.mailer.aws_route53_zone.this: aws_route53_zone.this: value of 'count' cannot be computed

N.B module version which encounters this error yet to be published (currently in development on my laptop)