Docs are misleading: HP m227fdn seems to need the plugin for scanning

[Thorsten Leemhuis]

I bought the m227fdn because
https://developers.hp.com/hp-linux-imaging-and-printing/supported_devices/index
stated it doesn't need the plugin, as I prefer not to have any non-free software on my system. I even verified that it's not listed on
https://developers.hp.com/hp-linux-imaging-and-printing/binary_plugin.html

But it seems I was mislead, as it didn't get it to work with; hp-scan for example reported:

> error: SANE: Error during device I/O (code=9)

I then ran /usr/bin/hp-diagnose_plugin and it told me I needed the plugin. It downloaded and installed it and then the scanner worked.