Merge #16397: doc: Clarify includeWatching for fundrawtransaction

8003104 Clarify includeWatching for fundrawtransaction (Steven Roose)

Pull request description:

  Might be sufficient to solve bitcoin/bitcoin#16396, bitcoin/bitcoin#7879 and bitcoin/bitcoin#14405.

ACKs for top commit:
  Sjors:
    ACK 8003104. This will always be confusing, but at least it gives a bunch more clues for the user to google.

Tree-SHA512: 9b8002c259c50f93d89fc5574105aae6152858d8d45c07b4c3d5b7023adafe73c7a98a290874ff3fbbb7dfad2ac1bdf4acb8769a2a1c14e38484922f44e84e54

Author: laanwj

src/wallet/rpcwallet.cpp

@@ -3135,7 +3135,9 @@ static UniValue fundrawtransaction(const JSONRPCRequest& request)
                             {"changeAddress", RPCArg::Type::STR, /* default */ "pool address", "The bitcoin address to receive the change"},
                             {"changePosition", RPCArg::Type::NUM, /* default */ "random", "The index of the change output"},
                             {"change_type", RPCArg::Type::STR, /* default */ "set by -changetype", "The output type to use. Only valid if changeAddress is not specified. Options are \"legacy\", \"p2sh-segwit\", and \"bech32\"."},
-                            {"includeWatching", RPCArg::Type::BOOL, /* default */ "true for watch-only wallets, otherwise false", "Also select inputs which are watch only"},
+                            {"includeWatching", RPCArg::Type::BOOL, /* default */ "true for watch-only wallets, otherwise false", "Also select inputs which are watch only.\n"
+                                                          "Only solvable inputs can be used. Watch-only destinations are solvable if the public key and/or output script was imported,\n"
+                                                          "e.g. with 'importpubkey' or 'importmulti' with the 'pubkeys' or 'desc' field."},
                             {"lockUnspents", RPCArg::Type::BOOL, /* default */ "false", "Lock selected unspent outputs"},
                             {"feeRate", RPCArg::Type::AMOUNT, /* default */ "not set: makes wallet determine the fee", "Set a specific fee rate in " + CURRENCY_UNIT + "/kB"},
                             {"subtractFeeFromOutputs", RPCArg::Type::ARR, /* default */ "empty array", "A json array of integers.\n"