Add caution callout for base64 encoding

Author: shannonxtreme

content/en/docs/concepts/security/secrets-good-practices.md

@@ -110,5 +110,8 @@ If you configure a Secret through a
 {{< glossary_tooltip text="manifest" term_id="manifest" >}}, with the secret
 data encoded as base64, sharing this file or checking it in to a source
 repository means the secret is available to everyone who can read the manifest.
+
+{{<caution>}}
 Base64 encoding is _not_ an encryption method, it provides no additional
 confidentiality over plain text.
+{{</caution>}}