Update docs for missing proxy subnet type

[rshriram]

Updating field comments and tests for the new proxy subnet type GLOBAL_MANAGED_PROXY that is not represented here as well as fixing some old doc comments.

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)

compute: added a new type `GLOBAL_MANAGED_PROXY` for the filed `purpose` in the resource `google_compute_subnetwork`

[modular-magician]

Oops! It looks like you're using an unknown release-note type in your changelog entries:

• REPLACEME

Please only use the types listed in https://github.com/GoogleCloudPlatform/magic-modules/blob/master/.ci/RELEASE_NOTES_GUIDE.md.

[modular-magician]

Hello! I am a robot. It looks like you are a community contributor. @zli82016, 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.

Terraform GA: Diff ( 4 files changed, 14 insertions(+), 14 deletions(-))
Terraform Beta: Diff ( 4 files changed, 14 insertions(+), 14 deletions(-))
TF Conversion: Diff ( 3 files changed, 5 insertions(+), 5 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2909
Passed tests 2606
Skipped tests: 302
Affected tests: 1

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

TestAccContainerAwsNodePool_BetaBasicHandWritten

Get to know how VCR tests work

[modular-magician]

$\textcolor{red}{\textsf{Tests failed during RECORDING mode:}}$
TestAccContainerAwsNodePool_BetaBasicHandWritten[Error message] [Debug log]

$\textcolor{red}{\textsf{Please fix these to complete your PR.}}$
View the build log or the debug log for each test

[zli82016]

LGTM. Thanks.