Clarify preferred btrfs-progs dependency and fix some minor inconsistencies

cc @vbatts

There's been some confusion here thinking that 3.8 is fine and that 3.16 should not be preferred (and that specifying `btrfs_noversion` is "OK" even with 3.16+), so I've rewritten this bit to hopefully clarify that we do in fact prefer 3.16.1+ and that lower versions are highly discouraged but supported where necessary.