dax : Fix documentation with respect to struct pages

The documentation for dax is not up to date with respect to the struct
page support available in some of the device drivers that utilize
it.

Signed-off-by: Stephen Bates <sbates@raithlin.com>
Acked-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>