doc: Add release notes for -avoidpartialspends

doc/release-notes-pr12257.md

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+Notable changes
+===============
+
+Coin selection
+--------------
+- A new `-avoidpartialspends` flag has been added (default=false). If enabled, the wallet will try to spend UTXO's that point at the same destination
+  together. This is a privacy increase, as there will no longer be cases where a wallet will inadvertently spend only parts of the coins sent to
+  the same address (note that if someone were to send coins to that address after it was used, those coins will still be included in future
+  coin selections).