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>

Author: lihongnan

Documentation/filesystems/dax.rst

@@ -23,11 +23,11 @@ on it as usual.  The `DAX` code currently only supports files with a block
 size equal to your kernel's `PAGE_SIZE`, so you may need to specify a block
 size when creating the filesystem.
 
-Currently 4 filesystems support `DAX`: ext2, ext4, xfs and virtiofs.
+Currently 5 filesystems support `DAX`: ext2, ext4, xfs, virtiofs and erofs.
 Enabling `DAX` on them is different.
 
-Enabling DAX on ext2
---------------------
+Enabling DAX on ext2 and erofs
+------------------------------
 
 When mounting the filesystem, use the ``-o dax`` option on the command line or
 add 'dax' to the options in ``/etc/fstab``.  This works to enable `DAX` on all files