Add description and uninstall restriction for "SystemLog"

[joeqian10]

Close #601

This may look stupid.🤣 Maybe we should not implement "SystemLog" as a plugin, but which needs modification from neo-core:

        public static void Log(string source, LogLevel level, object message)
        {
            foreach (ILogPlugin plugin in Plugin.Loggers)
                plugin.Log(source, level, message);
        }

[shargon]

Maybe you want to save the logs in a syslog server or similar, I think that it's not bad.

[ProDog]

Now SystemLog is not a plugin, so should we remove it from plugins?

[erikzhang]

I think you should not list it as a plugin. Because it is a part of neo-cli, not a plugin, although it implements the plugin interface.