mention guestbook API

Author: pdurbin

doc/release-notes/6.10-release-notes.md

@@ -19,7 +19,7 @@ Highlights for Dataverse 6.10 include:
 - Review datasets
 - Infrastructure: Payara has been upgraded from version 6 to 7
 - Infrastructure: Java has been upgraded from version 17 to 21
-- New and improved APIs
+- New and improved APIs, including filling in a guestbook when downloading files
 - Bug fixes
 
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