Documentation: Add comment block with variable structure in setproduct example

[jnearing-kabam]

In the examples here: https://www.terraform.io/docs/configuration/functions/setproduct.html#finding-combinations-for-for_each

There is the definition for network_subnets. It would be easier to grok this example if there was a comment block with the variable structure

Something that would look like

// Result of network_subnets would look something like this: 
// [
//   [
//     {
//       foo : bar
//       baz : bat
//     },
//     "test"
//   ],
//   [
//     {
//       foo : bat
//       baz : foo
//     },
//     "test-2"
//   ]
// ]

Not that the above is probably not the structure network_subnets provides. I have no idea what the actual structure is, which is why Im requesting the structure be made available somewhere.

[tr0njavolta]

Following up on this per @laurapacilio's request.

The output of the network_subnets locals value would look something like:

  {
    "cidr_block" = "10.1.16.0/20"
    "network_id" = "vpc-0bfb00ca6173ea5aa"
    "network_key" = "a"
    "subnet_key" = "a"
  },
  {
    "cidr_block" = "10.1.32.0/20"
    "network_id" = "vpc-0bfb00ca6173ea5aa"
    "network_key" = "a"
    "subnet_key" = "b"
  },
  {
    "cidr_block" = "10.1.48.0/20"
    "network_id" = "vpc-0bfb00ca6173ea5aa"
    "network_key" = "a"
    "subnet_key" = "c"
  },
  {
    "cidr_block" = "10.2.16.0/20"
    "network_id" = "vpc-0d193e011f6211a7d"
    "network_key" = "b"
    "subnet_key" = "a"
  },
  {
    "cidr_block" = "10.2.32.0/20"
    "network_id" = "vpc-0d193e011f6211a7d"
    "network_key" = "b"
    "subnet_key" = "b"
  },
  {
    "cidr_block" = "10.2.48.0/20"
    "network_id" = "vpc-0d193e011f6211a7d"
    "network_key" = "b"
    "subnet_key" = "c"
  },
]

Tested with a terraform.tfvars file with a map of two base cidr blocks and three subnets per availability zone.

If I'm understanding the configuration correctly, the setproduct function pairs the two maps from the networks & subnets locals values and creates a unique block of values to assign cidr_blocks from the base_cidr_block between the regions availability zones.

[laurapacilio]

Thank you so much @tr0njavolta! I'm creating a PR to add this to the official documentation as well. @jnearing-kabam - thank you for this feedback! Let us know if you have additional questions or suggestions.