rpc: Reject beginning newline in RPC docs

It is harmless, but a bit confusing and not needed for new code.

Also, update the remaining instances that were not found by the
scripted-diff.

src/rpc/node.cpp

@@ -273,8 +273,9 @@ static RPCHelpMan logging()
 
 static RPCHelpMan echo(const std::string& name)
 {
-    return RPCHelpMan{name,
-                "\nSimply echo back the input arguments. This command is for testing.\n"
+    return RPCHelpMan{
+        name,
+        "Simply echo back the input arguments. This command is for testing.\n"
                 "\nIt will return an internal bug report when arg9='trigger_internal_bug' is passed.\n"
                 "\nThe difference between echo and echojson is that echojson has argument conversion enabled in the client-side table in "
                 "bitcoin-cli and the GUI. There is no server-side difference.",

src/rpc/output_script.cpp

@@ -169,8 +169,9 @@ static RPCHelpMan getdescriptorinfo()
 {
     const std::string EXAMPLE_DESCRIPTOR = "wpkh([d34db33f/84h/0h/0h]0279be667ef9dcbbac55a06295Ce870b07029Bfcdb2dce28d959f2815b16f81798)";
 
-    return RPCHelpMan{"getdescriptorinfo",
-        {"\nAnalyses a descriptor.\n"},
+    return RPCHelpMan{
+        "getdescriptorinfo",
+        "Analyses a descriptor.\n",
         {
             {"descriptor", RPCArg::Type::STR, RPCArg::Optional::NO, "The descriptor."},
         },
@@ -260,8 +261,9 @@ static RPCHelpMan deriveaddresses()
 {
     const std::string EXAMPLE_DESCRIPTOR = "wpkh([d34db33f/84h/0h/0h]xpub6DJ2dNUysrn5Vt36jH2KLBT2i1auw1tTSSomg8PhqNiUtx8QX2SvC9nrHu81fT41fvDUnhMjEzQgXnQjKEu3oaqMSzhSrHMxyyoEAmUHQbY/0/*)#cjjspncu";
 
-    return RPCHelpMan{"deriveaddresses",
-        {"\nDerives one or more addresses corresponding to an output descriptor.\n"
+    return RPCHelpMan{
+        "deriveaddresses",
+        "Derives one or more addresses corresponding to an output descriptor.\n"
          "Examples of output descriptors are:\n"
          "    pkh(<pubkey>)                                     P2PKH outputs for the given pubkey\n"
          "    wpkh(<pubkey>)                                    Native segwit P2PKH outputs for the given pubkey\n"
@@ -270,7 +272,7 @@ static RPCHelpMan deriveaddresses()
          "    tr(<pubkey>,multi_a(<n>,<pubkey>,<pubkey>,...))   P2TR-multisig outputs for the given threshold and pubkeys\n"
          "\nIn the above, <pubkey> either refers to a fixed public key in hexadecimal notation, or to an xpub/xprv optionally followed by one\n"
          "or more path elements separated by \"/\", where \"h\" represents a hardened child key.\n"
-         "For more information on output descriptors, see the documentation in the doc/descriptors.md file.\n"},
+        "For more information on output descriptors, see the documentation in the doc/descriptors.md file.\n",
         {
             {"descriptor", RPCArg::Type::STR, RPCArg::Optional::NO, "The descriptor."},
             {"range", RPCArg::Type::RANGE, RPCArg::Optional::OMITTED, "If a ranged descriptor is used, this specifies the end or the range (in [begin,end] notation) to derive."},

src/rpc/server.cpp

@@ -156,11 +156,12 @@ static RPCHelpMan help()
 static RPCHelpMan stop()
 {
     static const std::string RESULT{CLIENT_NAME " stopping"};
-    return RPCHelpMan{"stop",
+    return RPCHelpMan{
+        "stop",
     // Also accept the hidden 'wait' integer argument (milliseconds)
     // For instance, 'stop 1000' makes the call wait 1 second before returning
     // to the client (intended for testing)
-                "\nRequest a graceful shutdown of " CLIENT_NAME ".",
+        "Request a graceful shutdown of " CLIENT_NAME ".",
                 {
                     {"wait", RPCArg::Type::NUM, RPCArg::Optional::OMITTED, "how long to wait in ms", RPCArgOptions{.hidden=true}},
                 },

src/rpc/util.cpp

@@ -1,4 +1,4 @@
-// Copyright (c) 2017-2022 The Bitcoin Core developers
+// Copyright (c) 2017-present The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -814,6 +814,7 @@ std::string RPCHelpMan::ToString() const
     if (was_optional) ret += " )";
 
     // Description
+    CHECK_NONFATAL(!m_description.starts_with('\n'));  // Historically \n was required, but reject it for new code.
     ret += "\n\n" + TrimString(m_description) + "\n";
 
     // Arguments