doc: Add release notes for 15620

doc/release-notes-15620.md

@@ -0,0 +1,13 @@
+Updated RPCs
+------------
+
+* The -maxtxfee setting no longer has any effect on non-wallet RPCs.
+
+  The `sendrawtransaction` and `testmempoolaccept` RPC methods previously
+  accepted an `allowhighfees` parameter to fail the mempool acceptance in case
+  the transaction's fee would exceed the value of the command line argument
+  `-maxtxfee`. To uncouple the RPCs from the global option, they now have a
+  hardcoded default for the maximum transaction fee, that can be changed for
+  both RPCs on a per-call basis with the `maxfeerate` parameter. The
+  `allowhighfees` boolean option has been removed and replaced by the
+  `maxfeerate` numeric option.

src/rpc/rawtransaction.cpp

@@ -39,6 +39,10 @@
 
 #include <univalue.h>
 
+/** High fee for sendrawtransaction and testmempoolaccept.
+ * By default, transaction with a fee higher than this will be rejected by the
+ * RPCs. This can be overriden with the maxfeerate argument.
+ */
 constexpr static CAmount DEFAULT_MAX_RAW_TX_FEE{COIN / 10};
 
 static void TxToJSON(const CTransaction& tx, const uint256 hashBlock, UniValue& entry)