[doc] release note effective-feerate and effective-includes RPC results

No release note for submitpackage because it is regtest-only.

release-notes-26646.md

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+JSON-RPC
+--------
+
+The `testmempoolaccept` RPC now returns 2 additional results within the "fees" result:
+"effective-feerate" is the feerate including fees and sizes of transactions validated together if
+package validation was used, and also includes any modified fees from prioritisetransaction. The
+"effective-includes" result lists the wtxids of transactions whose modified fees and sizes were used
+in the effective-feerate (#26646).