btrfs: update comment for btrfs_join_transaction_nostart()

Update the comment for btrfs_join_transaction_nostart() to be more clear
about how it works and how it's different from btrfs_attach_transaction().

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>