doc: update release notes for 22539

Following Marcofalke's feedback at bitcoin/bitcoin#22539 (comment)

Author: darosior

doc/release-notes-22539.md

@@ -61,6 +61,12 @@ P2P and network changes
   They will become eligible for address gossip after sending an ADDR, ADDRV2,
   or GETADDR message. (#21528)
 
+Fee estimation changes
+----------------------
+
+- Fee estimation now takes the feerate of replacement (RBF) transactions into
+  account. (#22539)
+
 Rescan startup parameter removed
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