[DOCS] Update kmip.mdx to add Cert Authority details (#23907)

* Update kmip.mdx to add Cert Authority details

* Update website/content/docs/secrets/kmip.mdx

Accepted suggestion

Co-authored-by: Sarah Chavis <62406755+schavis@users.noreply.github.com>

---------

Co-authored-by: Sarah Chavis <62406755+schavis@users.noreply.github.com>

website/content/docs/secrets/kmip.mdx

@@ -74,6 +74,15 @@ requests.
     ```text
     $ vault write kmip/config listen_addrs=0.0.0.0:5696
     ```
+### KMIP Certificate Authority for Client Certificates
+
+When the KMIP Secrets Engine is initially configured, Vault generates a KMIP 
+Certificate Authority (CA) whose only purpose is to authenticate KMIP client
+certificates.
+
+Vault uses the internal KMIP CA to generate certificates for clients
+authenticating to Vault with the KMIP protocol. You cannot import external KMIP
+authorities. All KMIP authentication must use the internally-generated KMIP CA.
 
 ## Usage