Documentation/dax: Update DAX enablement for ext4

Update the document to reflect ext4 and xfs now behave the same.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>

Link: https://lore.kernel.org/r/20200528150003.828793-10-ira.weiny@intel.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>