add docs for sa credential generation

[megan07]

Addresses hashicorp/terraform-provider-google#6497

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 ( 1 file changed, 4 insertions(+))
Terraform Beta: Diff ( 1 file changed, 4 insertions(+))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=119023"