dms/repl_subnet_grp: Fix inconsistent result after apply

[YakDriver]

Description

This resolves 3 different problems with aws_dms_replication_subnet_group:

1. Inconsistent result error: This happens because of a code bug. If there's a non-timeout error during creation, instead of returning the error, it ignores it. On the read, when there is no resource found, there's no check for it being not found.
2. Documenting the need to use subnets that cover 2 AZs and, thus, requiring at least 2 subnet IDs.
3. Documenting having the IAM role dms-vpc-role in place.

Relations

Closes #27420

References

Output from Acceptance Testing

% make testacc TESTS=TestAccDMSReplicationSubnetGroup_ PKG=dms
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/dms/... -v -count 1 -parallel 20 -run='TestAccDMSReplicationSubnetGroup_'  -timeout 180m
=== RUN   TestAccDMSReplicationSubnetGroup_basic
=== PAUSE TestAccDMSReplicationSubnetGroup_basic
=== CONT  TestAccDMSReplicationSubnetGroup_basic
--- PASS: TestAccDMSReplicationSubnetGroup_basic (42.90s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/dms	44.646s

[github-actions]

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[github-actions]

This functionality has been released in v4.50.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

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[github-actions]

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