Azure documentation needs to specify allowed locations

[jolcese]

Tried to deploy Tectonic on westus2 location by setting tectonic_azure_location and it failed with a error related to fault-domains.
When location changed to eastus, the deployment succeeded.

Error applying plan:

3 error(s) occurred:

* module.etcd.azurerm_availability_set.etcd: 1 error(s) occurred:

* azurerm_availability_set.etcd: compute.AvailabilitySetsClient#CreateOrUpdate: Failure responding to request: StatusCode=400 -- Original Error: autorest/azure: Service returned an error. Status=400 Code="InvalidParameter" Message="The specified fault domain count 3 must fall in the range 1 to 2."
* module.workers.azurerm_availability_set.tectonic_workers: 1 error(s) occurred:

* azurerm_availability_set.tectonic_workers: compute.AvailabilitySetsClient#CreateOrUpdate: Failure responding to request: StatusCode=400 -- Original Error: autorest/azure: Service returned an error. Status=400 Code="InvalidParameter" Message="The specified fault domain count 3 must fall in the range 1 to 2."
* module.masters.azurerm_availability_set.tectonic_masters: 1 error(s) occurred:

* azurerm_availability_set.tectonic_masters: compute.AvailabilitySetsClient#CreateOrUpdate: Failure responding to request: StatusCode=400 -- Original Error: autorest/azure: Service returned an error. Status=400 Code="InvalidParameter" Message="The specified fault domain count 3 must fall in the range 1 to 2."

Terraform does not automatically rollback in the face of errors.
Instead, your Terraform state file has been partially updated with
any resources that successfully completed. Please address the error
above and apply again to incrementally change your infrastructure.