update docs for TFC backend

[megan07]

Fixes hashicorp/terraform-provider-google#9917

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).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

[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, 14 insertions(+))
Terraform Beta: Diff ( 4 files changed, 17 insertions(+), 2 deletions(-))

[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.

[laurapacilio]

Suggested change

	3. Mark the variable as **Sensitive** and click **Save variable**.
	All runs within the workspace will use the `GOOGLE_CREDENTIALS` variable to authenticate with Google Cloud Platform.
	3. Mark the variable as **Sensitive** and click **Save variable**.
	All runs within the workspace will use the `GOOGLE_CREDENTIALS` variable to authenticate with Google Cloud Platform.

[laurapacilio]

Suggested change

	3. Mark the variable as **Sensitive** and click **Save variable**.
	All runs within the workspace will use the `GOOGLE_CREDENTIALS` variable to authenticate with Google Cloud Platform.
	3. Mark the variable as **Sensitive** and click **Save variable**.
	All runs within the workspace will use the `GOOGLE_CREDENTIALS` variable to authenticate with Google Cloud Platform.

[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.

[laurapacilio]

Suggested change

	3. Mark the variable as **Sensitive** and click **Save variable**.
	All runs within the workspace will use the `GOOGLE_CREDENTIALS` variable to authenticate with Google Cloud Platform.
	3. Mark the variable as **Sensitive** and click **Save variable**.
	All runs within the workspace will use the `GOOGLE_CREDENTIALS` variable to authenticate with Google Cloud Platform.

[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.

[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, 16 insertions(+))
Terraform Beta: Diff ( 3 files changed, 17 insertions(+), 2 deletions(-))

[laurapacilio]

@googlebot I fixed it.