ext3: update documentation with barrier=1 default

Commit 00eacd66 ("ext3: make ext3 mount default to barrier=1") changed
the default barrier mount option for ext3.  The documentation needs to
be updated, so this patch does that.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Jan Kara <jack@suse.cz>