Mention bumpfee in 0.14 release notes.

doc/release-notes.md

@@ -168,7 +168,8 @@ Opt into RBF When Sending
 
 A new startup option, `-walletrbf`, has been added to allow users to have all
 transactions sent opt into RBF support. The default value for this option is
-currently `false`, so transactions will not opt into RBF by default.
+currently `false`, so transactions will not opt into RBF by default. The new
+`bumpfee` RPC can be used to replace transactions that opt into RBF.
 
 Sensitive Data Is No Longer Stored In Debug Console History
 -----------------------------------------------------------
@@ -247,6 +248,11 @@ Low-level RPC changes
    optimizations to memory management. See [Pull #8753](https://github.com/bitcoin/bitcoin/pull/8753)
    for more information.
 
+ - A new RPC command `bumpfee` has been added which allows replacing an
+   unconfirmed wallet transaction that signaled RBF (see the `-walletrbf`
+   startup option above) with a new transaction that pays a higher fee, and
+   should be more likely to get confirmed quickly.
+
 HTTP REST Changes
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