Clarify the database resource name in firestore document surface

[pl04351820]

Clarify the database resource name in firestore document surface.

The issue was spot at hashicorp/terraform-provider-google#15550. Firestore document does not support databaseId in Terraform due to resource header constraint. Add a warning note section to clarify the beahvior.

[modular-magician]

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[modular-magician]

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Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 1 file changed, 2 insertions(+))
Terraform Beta: Diff ( 1 file changed, 2 insertions(+))