[DOCS] Add cross-link for other encryption key settings (elastic#165014)

Problem: The [Configure security in Kibana](https://www.elastic.co/guide/en/kibana/current/using-kibana-with-security.html) docs page only covers the `xpack.security.encryptionKey` setting for session encryption. Users may not know that encryption for Kibana's reporting and saved objects features also require encryption keys.

Solution: Add a cross-link to the respective encryption key settings for  reporting and saved objects

docs/user/security/securing-kibana.asciidoc

@@ -27,15 +27,18 @@ configure additional security settings and authentication.
 
 . Set the `xpack.security.encryptionKey` property in the `kibana.yml`
 configuration file. You can use any text string that is 32 characters or longer
-as the encryption key.
+as the encryption key. Refer to <<xpack-security-encryptionKey,`xpack.security.encryptionKey`>>.
 +
 --
 [source,yaml]
 ----
 xpack.security.encryptionKey: "something_at_least_32_characters"
 ----
 
-For more information, see <<security-settings-kb,Security settings in {kib}>>.
+{kib}'s reporting and saved objects features also have encryption key settings.
+Refer to <<xpack-reporting-encryptionKey,`xpack.reporting.encryptionKey`>> and
+<<xpack-encryptedSavedObjects-encryptionKey,`xpack.encryptedSavedObjects.encryptionKey`>>
+respectively.
 --
 
 . Optional: <<xpack-security-session-management,Configure {kib}'s session expiration settings>>.