[sshfs] Add call once comment to make_quit_watchdog

include/multipass/platform.h

@@ -96,6 +96,7 @@ int symlink_attr_from(const char* path, sftp_attributes_struct* attr);
 // Creates a function that will wait for signals or until the passed function returns false.
 // The passed function is checked every `period` milliseconds.
 // If a signal is received the optional contains it, otherwise the optional is empty.
+// `make_quit_watchdog` should only be called once.
 std::function<std::optional<int>(const std::function<bool()>&)> make_quit_watchdog(
     const std::chrono::milliseconds& period); // call while single-threaded; call result later, in dedicated thread