On the off chance that somebody has time and a USB scanner that is
supported by SANE, please update the Scanner chapter  in the
handbook. It has gotten a little stale. sane-frontends is
deprecated, recommending the unmaintained xsane might not be
the best idea. graphics/kooka might be an alternative to xsane.
Advice in "7.7.3 Scanner Permissions" is bad too, because it uses
devfs.rules and assumes that USB device nodes never change.
Permissions change should happen via devd instead based on scanner
vendor/product id. Somebody has already written a rough outline
about this 10 years ago in sane-backends's pkg-message  but it
could use some polishing and IMHO it would be much better to have
it in the Handbook.
Ideally I'd like to completely remove or trim the pkg-message from
sane-backends a lot because I do not think a page long tutorial
I'd like to minimize the mention to frontends, since many people may use a desktop env. coming, I assume, with its own frontend. Just mentioning both kooka and Xsane (because of Gimp) may be enough.
add path 'usb/*' mode 0660 group usb
add path 'ugen*' mode 0666 group usb
may be better. Then, we can also add an example using devd.
I'm also wondering if we need to keep the part on SCSI.
Created attachment 212078 [details]
Here's my first, incomplete, work.
I updated several things (see the "First pass" patch for more details).
I'm still not sure about devd(8) since it's more complex for the user than just using devfs(8).
See https://people.freebsd.org/~blackend/scanners/scanners.html for a rendered version.