Document new serial_number_source PKI role field (#29438)

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

@@ -3475,6 +3475,13 @@ request is denied.
   certificates are stored. If true, metadata is not stored and an error is returned
   if the `metadata` field is specified on issuance APIs
 
+- `serial_number_source` `(string: "json-csr")` - Specifies the source of the subject serial
+  number. Valid values are `json-csr` (default) or `json`.
+  When set to `json-csr`, the subject serial number is taken from the `serial_number` parameter
+  and falls back to the serial number in the CSR. When set to `json`, the subject serial number
+  is taken from the `serial_number` parameter but will ignore any value in the CSR.
+  For backwards compatibility an empty value for this field will default to the `json-csr` behavior.
+
 #### Sample payload
 
 ```json