[docs] Add release notes for PR 15427

Author: jnewbery

doc/release-notes-15427.md

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+Updated RPCs
+------------
+
+The `utxoupdatepsbt` RPC method has been updated to take a `descriptors`
+argument. When provided, input and output scripts and keys will be filled in
+when known, and P2SH-witness inputs will be filled in from the UTXO set when a
+descriptor is provided that shows they're spending segwit outputs.
+
+See the RPC help text for full details.