Backport of docs: update splat expressions in directives into v1.5

[teamterraform]

Backport

This PR is auto-generated from #32074 to be assessed for backporting due to the inclusion of the label 1.5-backport.

The below text is copied from the body of the original PR.

---

Updates the Expressions > Strings and Templates docs to use the recommend splat expression syntax, with square brackets. While Terraform continues to support the .* syntax for resources with count, the docs describe them as "legacy 'attribute-only' splat expressions."

For good measure, I tested to confirm this works:

output "ips" {
  value = <<-EOF
    %{ for ip in aws_instance.example[*].private_ip }
    server ${ip}
    %{ endfor }
  EOF
}

resource "aws_instance" "example" {
  count = 2
  instance_type = "t3.nano"
  ami = "ami-0d593311db5abb72b"
}

Apply complete! Resources: 2 added, 0 changed, 0 destroyed.

Outputs:

ips = <<EOT

server 172.31.52.27

server 172.31.53.123


EOT

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

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