btrfs: update documentation for BTRFS_RESERVE_FLUSH_EVICT flush method

The BTRFS_RESERVE_FLUSH_EVICT flush method can also commit transactions,
see the definition of the evict_flush_states const array at space-info.c,
but the documentation for it at space-info.h does not mention it.
So update the documentation.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>