[DOCS] Adds clarification ot encryption key setting (#51244) (#51329)

docs/settings/security-settings.asciidoc

@@ -40,6 +40,8 @@ An arbitrary string of 32 characters or more that is used to encrypt credentials
 in a cookie. It is crucial that this key is not exposed to users of {kib}. By
 default, a value is automatically generated in memory. If you use that default
 behavior, all sessions are invalidated when {kib} restarts.
+In addition, high-availability deployments of {kib} will behave unexpectedly 
+if this setting isn't the same for all instances of {kib}.
 
 `xpack.security.secureCookies`::
 Sets the `secure` flag of the session cookie. The default value is `false`. It