btrfs-progs: document snapshot unaware defrag

btrfs-progs: document snapshot unaware defrag

In btrfs-filesystem(8), improved the documentation of snapshot unaware
defragmentation and included the exact kernel version numbers being affected as
well as the possible effects.
No longer use the word "unlink" which is easily understood as "deleting a file".
Moved the warning more to the beginning of "defragment" subcommand's
documentation where it's more visible to readers.

Added the same warning to the "autodefrag" option of btrfs-mount(5).

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>