doc: add release note for bitcoin#27460 (new importmempool RPC)

Co-authored-by: glozow <gloriajzhao@gmail.com>

doc/release-notes-27460.md

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+- A new `importmempool` RPC has been added. It loads a valid `mempool.dat` file and attempts to
+  add its contents to the mempool. This can be useful to import mempool data from another node
+  without having to modify the datadir contents and without having to restart the node.
+    - Warning: Importing untrusted files is dangerous, especially if metadata from the file is taken over.
+    - If you want to apply fee deltas, it is recommended to use the `getprioritisedtransactions` and
+      `prioritisetransaction` RPCs instead of the `apply_fee_delta_priority` option to avoid duplicates.