dax: some small updates to dax.txt documentation

dax: some small updates to dax.txt documentation

These are originally from Matthew Wilcox and were part of his huge
"mm,fs,dax: Change ->pmd_fault to ->huge_fault" patch that was part of
PUD support.

I'm breaking these small changes out as they stand on their own and add
useful information to Documentation/filesystems/dax.txt.

Link: http://lkml.kernel.org/r/20160714214049.20075-1-ross.zwisler@linux.intel.com
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jan Kara <jack@suse.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>