Clarify format of limit string in service quota override resources

[modular-magician]

Description

Relates to this issue : hashicorp/terraform-provider-google#12253

The google_service_usage_consumer_quota_override resource uses a different version of the customer quota metrics API than the gcloud CLI, and the format of the units used by the override differs between each API version.

This PR:

• Adds guidance to the resource's documentation, to help users avoid the issue
• Adds a test that asserts that the 1/{project}/{user} format style is incorrect for the provider, to help people understand expected behaviour

I decided against adding validation on the limit field because what is 'wrong' now will eventually be correct once the resource is updated to use the newer API

List

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)

Derived from GoogleCloudPlatform/magic-modules#6396