[Doc] importmulti: add missing description of keypool option

src/wallet/rpcdump.cpp

@@ -1382,6 +1382,7 @@ UniValue importmulti(const JSONRPCRequest& mainRequest)
                                     {"internal", RPCArg::Type::BOOL, /* default */ "false", "Stating whether matching outputs should be treated as not incoming payments (also known as change)"},
                                     {"watchonly", RPCArg::Type::BOOL, /* default */ "false", "Stating whether matching outputs should be considered watchonly."},
                                     {"label", RPCArg::Type::STR, /* default */ "''", "Label to assign to the address, only allowed with internal=false"},
+                                    {"keypool", RPCArg::Type::BOOL, /* default */ "false", "Stating whether imported public keys should be added to the keypool for when users request new addresses. Only allowed when wallet private keys are disabled"},
                                 },
                             },
                         },