Add PKI API documentation on cn_validations

Signed-off-by: Alexander Scheel <[email protected]>

website/content/api-docs/secret/pki.mdx

@@ -2419,6 +2419,19 @@ request is denied.
   `YYYY-MM-ddTHH:MM:SSZ`. Supports the Y10K end date for IEEE 802.1AR-2018
   standard devices, `9999-12-31T23:59:59Z`.
 
+- `cn_validations` `(list: ["email", "hostname"])` - Validations to run on the
+  Common Name field of the certificate. Valid values include:
+
+   - `email`, to ensure the Common Name is an email address (contains an `@` sign),
+   - `hostname`, to ensure the Common Name is a hostname (otherwise).
+
+  Multiple values can be separated with a comma or specified as a list and use
+  OR semantics (either email or hostname in the CN are allowed). When the
+  special value "disabled" is used (must be specified alone), none of the usual
+  validation is run (including but not limited to `allowed_domains` and basic
+  correctness validation around email addresses and domain names). This allows
+  non-standard CNs to be used verbatim from the request.
+
 #### Sample Payload
 
 ```json