x/sys: improve NewCallback documentation

windows/syscall_windows.go

@@ -112,12 +112,14 @@ func Getpagesize() int { return 4096 }
 
 // NewCallback converts a Go function to a function pointer conforming to the stdcall calling convention.
 // This is useful when interoperating with Windows code requiring callbacks.
+// The argument is expected to be a function with with one uintptr-sized result. The function must not have arguments with size larger than the size of uintptr.
 func NewCallback(fn interface{}) uintptr {
 	return syscall.NewCallback(fn)
 }
 
 // NewCallbackCDecl converts a Go function to a function pointer conforming to the cdecl calling convention.
 // This is useful when interoperating with Windows code requiring callbacks.
+// The argument is expected to be a function with with one uintptr-sized result. The function must not have arguments with size larger than the size of uintptr.
 func NewCallbackCDecl(fn interface{}) uintptr {
 	return syscall.NewCallbackCDecl(fn)
 }