[doc] add release note for package testmempoolaccept

doc/release-notes-20833.md

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+Updated RPCs
+------------
+
+- The `testmempoolaccept` RPC now accepts multiple transactions (still experimental at the moment,
+  API may be unstable). This is intended for testing transaction packages with dependency
+  relationships; it is not recommended for batch-validating independent transactions. In addition to
+  mempool policy, package policies apply: the list cannot contain more than 25 transactions or have a
+  total size exceeding 101K virtual bytes, and cannot conflict with (spend the same inputs as) each other or
+  the mempool, even if it would be a valid BIP125 replace-by-fee. There are some known limitations to
+  the accuracy of the test accept: it's possible for `testmempoolaccept` to return "allowed"=True for a
+  group of transactions, but "too-long-mempool-chain" if they are actually submitted. (#20833)
+