provider/aws: Only send iops when creating io1 devices. Fix docs

[catsby]

I think #11619 revealed a "bug" in our tests where iops were being specified in a gp2 volume. Because the root_block_device hash was always zero (it couldn't be changed), we failed to detect this.

Fixes TestAccAWSInstance_GP2IopsDevice test error:

------- Stdout: -------
=== RUN   TestAccAWSInstance_GP2IopsDevice
--- FAIL: TestAccAWSInstance_GP2IopsDevice (168.38s)
    testing.go:265: Step 0 error: After applying this step, the plan was not empty:
        
        DIFF:
        
        DESTROY/CREATE: aws_instance.foo
          associate_public_ip_address:               "true" => "<computed>"
          availability_zone:                         "us-west-2c" => "<computed>"
          ebs_block_device.#:                        "0" => "<computed>"
          ephemeral_block_device.#:                  "0" => "<computed>""
          root_block_device.#:                       "1" => "1"
          root_block_device.0.delete_on_termination: "true" => "true"
          root_block_device.0.iops:                  "100" => "330" (forces new resource)

Test runs:

TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSInstance_GP2IopsDevice -timeout 120m
=== RUN   TestAccAWSInstance_GP2IopsDevice
--- PASS: TestAccAWSInstance_GP2IopsDevice (59.02s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	59.051s

TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSInstance_blockDevices -timeout 120m
=== RUN   TestAccAWSInstance_blockDevices
--- PASS: TestAccAWSInstance_blockDevices (85.59s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	85.613s

[ghost]

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