doc: release note for 30212

doc/release-notes-30212.md

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+RPC
+---
+
+- Previously when using the `sendrawtransaction` rpc and specifying outputs
+  that are already in the UXTO set an RPC error code `-27` with RPC error
+  text "Transaction already in block chain" was returned in response.
+  The help text has been updated to "Transaction outputs already in utxo set"
+  to more accurately describe the source of the issue.