wallet: update mempool conflicts tests + docs

doc/release-notes-27307.md

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+Wallet
+---
+
+The wallet now detects when wallet transactions conflict with the mempool. Mempool
+conflicting transactions can be seen in the `"mempoolconflicts"` field of
+`gettransaction`. The inputs of mempool conflicted transactions can now be respent
+without manually abandoning the transactions when the parent transaction is dropped
+from the mempool, which can cause wallet balances to appear higher.