Btrfs: add a comment explaining what btrfs_cont_expand does

Everytime I have to deal with btrfs_cont_expand I stare at it for 20 minutes
trying to remember what exactly it does and why the hell we need it.  So add a
comment to save future-Josef some time.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
1 file changed