Start documenting the ioctl interface to btrfs. The overall structure
should be settled, the formatting of the ioctl description may change in
the future, newly added ioctl descriptions should follow the examples of
BTRFS_IOC_SUBVOL_CREATE.
The document is not finished yet and will not be installed until most of
ioctls' details are filled in.
Signed-off-by: David Sterba <dsterba@suse.com>