doc: add release notes

doc/release-notes-34075.md

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+Updated RPCs
+------------
+
+- The `estimatesmartfee` RPC now combines two fee rate estimators: the existing
+  block policy fee rate estimator and a new mempool fee rate estimator.
+
+- The new mempool fee rate estimator produces conservative and economical fee
+  rate estimates from the current contents of the mempool. It only produces a
+  fee rate estimate when recent blocks indicate a healthy mempool, and falls
+  back to the higher of the minimum relay fee rate and the current mempool
+  minimum fee rate when the mempool is too sparse. Its statistics are persisted
+  to `fees/mempool_policy_estimator.dat` and reloaded on startup.
+
+  `estimatesmartfee` returns the lower of the two fee rate estimators' results,
+  so the mempool fee rate estimator can only lower the block policy fee rate
+  estimate.
+
+- The combined estimate requires both estimators to succeed. If the mempool fee
+  rate estimator cannot produce an estimate, for example, while the mempool is
+  still loading, when too few recent blocks have been observed, or when the
+  mempool is suspected to be unhealthy, an error is returned.
+
+- `estimatesmartfee` accepts an `options` object with `fee_rate_estimator`.
+  Recognized values are `"none"` (the default, combined behavior described
+  above), `"block_policy"` (use only the block policy fee rate estimator),
+  and `"mempool_policy"` (use only the mempool fee rate estimator).
+  All unknown values are treated as `"none"`.
+  Users who want the previous behavior can select the block policy fee rate
+  estimator explicitly.
+
+- The options object also accepts `verbosity`. A verbosity of `2` or higher
+  also returns `mempool_health_statistics`.
+
+- When `fee_rate_estimator` is `"none"` and the estimate succeeds, the response
+  also includes an `estimator` field identifying which fee rate estimator produced
+  the result.
+
+- Block policy fee estimator data is now stored in
+  `fees/block_policy_estimates.dat`. If the new file does not exist, the
+  legacy `fee_estimates.dat` file is moved to the new path during startup. If
+  both files exist, the legacy file is removed.
+
+- Wallet fee rate estimation uses the default combined estimate.
+
+Wallet
+------
+
+- The `fee_reason` field returned by wallet transaction creation RPCs now
+  reports the reason the wallet selected the fee rate (fee rate estimator,
+  mempool minimum, fallback, or minimum required) instead of the block policy
+  fee rate estimator's internal threshold details. Those details remain
+  available in the block policy fee rate estimator debug log.