Documentation/filesystem/dax: update DAX description on erofs

Add missing erofs fsdax description since fsdax has been supported
on erofs from Linux 5.15.

Link: https://lore.kernel.org/r/20220308034139.93748-1-hongnan.li@linux.alibaba.com


Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: lihongnan <hongnan.lhn@alibaba-inc.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>