doc: release notes for avoid_reuse

doc/release-notes-13756.md

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+Coin selection
+--------------
+
+### Reuse Avoidance
+
+A new wallet flag `avoid_reuse` has been added (default off). When enabled,
+a wallet will distinguish between used and unused addresses, and default to not
+use the former in coin selection.
+
+(Note: rescanning the blockchain is required, to correctly mark previously
+used destinations.)
+
+Together with "avoid partial spends" (present as of Bitcoin v0.17), this
+addresses a serious privacy issue where a malicious user can track spends by
+peppering a previously paid to address with near-dust outputs, which would then
+be inadvertently included in future payments.
+
+New RPCs
+--------
+
+- A new `setwalletflag` RPC sets/unsets flags for an existing wallet.
+
+
+Updated RPCs
+------------
+
+Several RPCs have been updated to include an "avoid_reuse" flag, used to control
+whether already used addresses should be left out or included in the operation.
+These include:
+
+- createwallet
+- getbalance
+- sendtoaddress
+
+In addition, `sendtoaddress` has been changed to enable `-avoidpartialspends` when
+`avoid_reuse` is enabled.
+
+The listunspent RPC has also been updated to now include a "reused" bool, for nodes
+with "avoid_reuse" enabled.