libnvdimm, btt: update the usage section in Documentation

Section 5 about BTT's in kernel usage was quite obsolete,
replace it with a simple 'Usage' section that describes how
to set up a BTT namespace using the 'ndctl' utility.

Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>