Mark send RPC experimental

doc/release-notes-16378.md

@@ -1,5 +1,6 @@
 RPC
 ---
 - A new `send` RPC with similar syntax to `walletcreatefundedpsbt`, including
-  support for coin selection and a custom fee rate. Using the new `send` method
-  is encouraged: `sendmany` and `sendtoaddress` may be deprecated in a future release.
+  support for coin selection and a custom fee rate. The `send` RPC is experimental
+  and may change in subsequent releases. Using it is encouraged once it's no
+  longer experimental: `sendmany` and `sendtoaddress` may be deprecated in a future release.

src/wallet/rpcwallet.cpp

@@ -3874,6 +3874,7 @@ static UniValue listlabels(const JSONRPCRequest& request)
 static RPCHelpMan send()
 {
     return RPCHelpMan{"send",
+        "\nEXPERIMENTAL warning: this call may be changed in future releases.\n"
         "\nSend a transaction.\n",
         {
             {"outputs", RPCArg::Type::ARR, RPCArg::Optional::NO, "a json array with outputs (key-value pairs), where none of the keys are duplicated.\n"