enhance subnetwork resource with PRIVATE_NAT purpose in docs and test

[felipegc]

Fixes: hashicorp/terraform-provider-google#15833

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read Write release notes before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

[modular-magician]

Hello! I am a robot. It looks like you are a: Community Contributor Googler Core Contributor. Tests will require approval to run.

@SarahFrench, a repository maintainer, has been assigned to review your changes. If you have not received review feedback within 2 business days, please leave a comment on this PR asking them to take a look.

You can help make sure that review is quick by doing a self-review and by running impacted tests locally.

[felipegc]

@SarahFrench is that possible to start reviewing this code? Thanks

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 2 files changed, 30 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 3 files changed, 76 insertions(+), 2 deletions(-))
TF OiCS: Diff ( 4 files changed, 117 insertions(+))

[modular-magician]

Tests analytics

Total tests: 3081
Passed tests 2778
Skipped tests: 299
Affected tests: 4

Action taken

Found 4 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit. Click here to see the affected tests

TestAccComputeSubnetwork_subnetworkPurposePrivateNatExample|TestAccSpannerDatabaseIamPolicy|TestAccWorkstationsWorkstationConfig_update|TestAccWorkstationsWorkstationConfig_updateHostDetails

Get to know how VCR tests work

[modular-magician]

$\textcolor{green}{\textsf{Tests passed during RECORDING mode:}}$
TestAccComputeSubnetwork_subnetworkPurposePrivateNatExample[Debug log]
TestAccSpannerDatabaseIamPolicy[Debug log]
TestAccWorkstationsWorkstationConfig_update[Debug log]
TestAccWorkstationsWorkstationConfig_updateHostDetails[Debug log]

Rerun these tests in REPLAYING mode to catch issues

$\textcolor{green}{\textsf{No issues found for passed tests after REPLAYING rerun.}}$

---

$\textcolor{green}{\textsf{All tests passed!}}$
View the build log or the debug log for each test

[SarahFrench]

Looks great, thank you for making this change!

Also, I updated the release note to be empty, as we tend to not include test/documentation changes in changelogs.