Document default values for max_retry and destroy_max_retry

[dickon]

Please document the defaults for max_retry and destroy_max_retry. I believe from the source they are both zero, which means no retries by default. I expected to find this in the docs but had to go to the source code.

It may also be worth setting up a non-zero default so that network errors don't necessary cause terraform apply failures, despite the surprisingness of changing defaults. But I think in general terraform providers are expected to make a reasonable effort to keep going to reach the target state?