Document allowed_domains_template. (#9751)

website/pages/api-docs/secret/pki/index.mdx

@@ -755,6 +755,9 @@ request is denied.
 - `allowed_domains` `(list: [])` – Specifies the domains of the role. This is
   used with the `allow_bare_domains` and `allow_subdomains` options.
 
+- `allowed_domains_template` `()bool: false)` – When set, `allowed_domains`
+  may contain templates, as with [ACL Path Templating](/docs/concepts/policies).
+
 - `allow_bare_domains` `(bool: false)` – Specifies if clients can request
   certificates matching the value of the actual domains themselves; e.g. if a
   configured domain set with `allowed_domains` is `example.com`, this allows