Update google_iap_brand documentation about import and name formats

[modular-magician]

Closes #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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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).
• 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

Derived from GoogleCloudPlatform/magic-modules#6586

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