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.",