index: mention Btrfs in --no-check-device docs
index: mention Btrfs in --no-check-device docs

Thanks to Karl-Philipp Richter for the suggestion.

Signed-off-by: Rob Browning <1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@defaultvalue.org>

tjanez/bup,tjanez/bup,ToxicFrog/bup,gevaerts/bup,ToxicFrog/bup,gevaerts/bup,tjanez/bup,ToxicFrog/bup,ToxicFrog/bup,gevaerts/bup,tjanez/bup,gevaerts/bup,tjanez/bup,gevaerts/bup,tjanez/bup,ToxicFrog/bup