FixDoc Update subnetwork purpose description

[tferazzi]

Update the description of subnetwork purpose parameter.
API docs reference the purpose PRIVATE while the terraform-provider still mentions PRIVATE_RFC_1918. Specifying PRIVATE_RFC_1918 does not result in an error but causes the subnetwork to be re-created on every apply.

Fixes hashicorp/terraform-provider-google#16539

Release Note Template for Downstream PRs (will be copied)

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[github-actions]

Hello! I am a robot. Tests will require approval from a repository maintainer to run.

@NickElliot, 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.

[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.

google provider: Diff ( 2 files changed, 10 insertions(+), 8 deletions(-))
google-beta provider: Diff ( 2 files changed, 10 insertions(+), 8 deletions(-))

[modular-magician]

Tests analytics

Total tests: 1002
Passed tests: 926
Skipped tests: 75
Affected tests: 1

Click here to see the affected service packages

• compute

Action taken

Found 1 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit. Click here to see the affected tests

• TestAccComputeInstance_confidentialHyperDiskBootDisk

Get to know how VCR tests work

[modular-magician]

$\textcolor{green}{\textsf{Tests passed during RECORDING mode:}}$
TestAccComputeInstance_confidentialHyperDiskBootDisk[Debug log]

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

---

$\textcolor{green}{\textsf{All tests passed!}}$

View the build log or the debug log for each test

[github-actions]

@NickElliot This PR has been waiting for review for 3 weekdays. Please take a look! Use the label disable-review-reminders to disable these notifications.

[NickElliot]

Hi, in this case we don't want to alter the actual field behavior because that is adding a maintenance cost for a field which has proven it will change accepted values. Just updating our documentation of what is accepted/not is fine. Thanks for making the update!

[NickElliot]

Suggested change

	values:
	- :PRIVATE
	- :REGIONAL_MANAGED_PROXY
	- :GLOBAL_MANAGED_PROXY
	- :PRIVATE_SERVICE_CONNECT
	- :PRIVATE_NAT

[tferazzi]

Hi, thanks for your review. I removed the commit containing the var change and only kept the documentation updates.

[NickElliot]

Suggested change

	- !ruby/object:Api::Type::Enum
	- !ruby/object:Api::Type::String

[github-actions]

@NickElliot This PR has been waiting for review for 3 weekdays. Please take a look! Use the label disable-review-reminders to disable these notifications.

[NickElliot]

LGTM, thanks!