docs: add write_only field in field_references (GoogleCloudPlatform#13347)

Co-authored-by: Riley Karson <[email protected]>

Author: 2 people

docs/content/reference/field.md

@@ -98,6 +98,23 @@ Example:
 sensitive: true
 ```
 
+### `write_only`
+If true, the field is considered "write-only", which means that its value will
+be obscured in Terraform output as well as not be stored in state. This field is meant to replace `sensitive` as it doesn't store the value in state.
+See [Ephemerality in Resources - Use Write-only arguments](https://developer.hashicorp.com/terraform/language/resources/ephemeral/write-only)
+for more information.
+
+Write-only fields are only supported in Terraform v1.11+. Because the provider supports earlier Terraform versions, write only fields must be paired with (mutually exclusive) `sensitive` fields covering the same functionality for compatibility with those older versions.
+This field cannot be used in conjuction with `immutable` or `sensitive`.
+
+**Note**: Due to write-only not being read from the API, it is not possible to update the field directly unless a sidecar field is used. (e.g. `password` as a write-only field and `password_wo_version` as an immutable field meant for updating).
+
+Example:
+
+```yaml
+write_only: true
+```
+
 ### `ignore_read`
 If true, the provider sets the field's value in the resource state based only
 on the user's configuration. If false or unset, the provider sets the field's