release notes for fee estimation changes

doc/release-notes.md

@@ -131,6 +131,17 @@ of a node, among which:
 - Reduce the number of threads (#5964, #5679); lowers the amount of (esp.
   virtual) memory needed
 
+Fee estimation changes
+----------------------
+
+This release improves the algorithm used for fee estimation.  Previously, -1
+was returned when there was insufficient data to give an estimate.  Now, -1
+will also be returned when there is no fee or priority high enough for the
+desired confirmation target. In those cases, it can help to ask for an estimate
+for a higher target number of blocks. It is not uncommon for there to be no
+fee or priority high enough to be reliably (85%) included in the next block and
+for this reason, the default for `-txconfirmtarget=n` has changed from 1 to 2.
+
 Privacy: Disable wallet transaction broadcast
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