doc: add release notes snippet for cluster mempool

doc/release-notes-33629.md

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+Mempool
+=======
+
+The mempool has been reimplemented with a new design ("cluster mempool"), to
+facilitate better decision-making when constructing block templates, evicting
+transactions, relaying transactions, and validating replacement transactions
+(RBF). Most changes should be transparent to users, but some behavior changes
+are noted:
+
+- The mempool no longer enforces ancestor or descendant size/count limits.
+  Instead, two new default policy limits are introduced governing connected
+  components, or clusters, in the mempool, limiting clusters to 64 transactions
+  and up to 101 kB in virtual size.  Transactions are considered to be in the
+  same cluster if they are connected to each other via any combination of
+  parent/child relationships in the mempool. These limits can be overridden
+  using command line arguments; see the extended help (`-help-debug`)
+  for more information.
+
+- Within the mempool, transactions are ordered based on the feerate at which
+  they are expected to be mined, which takes into account the full set, or
+  "chunk", of transactions that would be included together (e.g., a parent and
+  its child, or more complicated subsets of transactions). This ordering is
+  utilized by the algorithms that implement transaction selection for
+  constructing block templates; eviction from the mempool when it is full; and
+  transaction relay announcements to peers.
+
+- The replace-by-fee validation logic has been updated so that transaction
+  replacements are only accepted if the resulting mempool's feerate diagram is
+  strictly better than before the replacement. This eliminates all known cases
+  of replacements occurring that make the mempool worse off, which was possible
+  under previous RBF rules. For singleton transactions (that are in clusters by
+  themselves) it's sufficient for a replacement to have a higher fee and
+  feerate than the original. See
+  [delvingbitcoin.org post](https://delvingbitcoin.org/t/an-overview-of-the-cluster-mempool-proposal/393#rbf-can-now-be-made-incentive-compatible-for-miners-11)
+  for more information.
+
+- Two new RPCs have been added: `getmempoolcluster` will provide the set of
+  transactions in the same cluster as the given transaction, along with the
+  ordering of those transactions and grouping into chunks; and
+  `getmempoolfeeratediagram` will return the feerate diagram of the entire
+  mempool.
+
+- Chunk size and chunk fees are now also included in the output of `getmempoolentry`.