Update google_iap_brand documentation about import and name formats

[SarahFrench]

Closes hashicorp/terraform-provider-google#12600

PR makes the two possible import formats for the google_iap_brand resource more explicit.

I also updated the description of the name attribute as I thought it'd be good to make it clear that it can take the two forms (but I think API returns project number for Read operations). Relevant documentation : https://cloud.google.com/iap/docs/reference/rest/v1/projects.brands/get#body.PATH_PARAMETERS.name

When I generated the downstream locally I saw that a third import format is added automatically : {{project_number}}/{{brand_id}} I couldn't figure out how to stop this happening, so I implemented the ability to import with that format of ID.

I currently don't know how we'd make acceptance tests to test import with the different import formats, because we'd require knowledge of the the project number of the project made in the acceptance test.

For now, I've tested the import manually. See image below.

1. Removing the resource from state
2. Attempting import with the latest provider release
3. Attempting import with developer override using a build of this PR
   • The resource in state has an id of {{project}}/{{brand_id}} but the name is set as projects/{{project}}/brands/{{brand_id}}

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• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• 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). N/A
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know). N/A
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

iap: added ability to import `google_iap_brand` using ID in {{project}}/{{brand_id}} format

[modular-magician]

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[SarahFrench]

If there's a simple solution to stop the downstream having this third import format added to the docs I'm happy to remove the new import code, but I figured implementing it would be easier!

$ terraform import google_iap_brand.default projects/{{project_id}}/brands/{{brand_id}}
$ terraform import google_iap_brand.default projects/{{project_number}}/brands/{{brand_id}}
- $ terraform import google_iap_brand.default {{project_number}}/{{brand_id}}

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 31 insertions(+), 12 deletions(-))
Terraform Beta: Diff ( 2 files changed, 31 insertions(+), 12 deletions(-))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2176
Passed tests 1936
Skipped tests: 238
Failed tests: 2

Action taken

Triggering VCR tests in RECORDING mode for the tests that failed during VCR. Click here to see the failed tests

TestAccComputeInstance_soleTenantNodeAffinities|TestAccCGCSnippet_eventarcWorkflowsExample

[modular-magician]

Tests failed during RECORDING mode:
TestAccComputeInstance_soleTenantNodeAffinities[Error message] [Debug log]
TestAccCGCSnippet_eventarcWorkflowsExample[Error message] [Debug log]

Please fix these to complete your PR
View the build log or the debug log for each test

[SarahFrench]

Thanks!