doc: add release notes for the openafsServer.authentication options

nixos/doc/manual/release-notes/rl-2505.section.md

@@ -31,6 +31,6 @@
 
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-- Create the first release note entry in this section!
+- The OpenAFS server module now supports the automatic extraction of service authentication keys from a Kerberos 5 keytab. Just specify the runtime path of the keytab and (optionally) the principal representing the AFS cell as the values of `services.openafsServer.authentication.keytab` and `services.openafsServer.authentication.principal`, respectively. The old behavior of using a pre-existing `/etc/openafs/server/KeyFileExt` managed by the `asetkey` command is retained by default.
 
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