doc: fix missing NVS API documentation

Sphinx only displays API documentation for items within the specified
doxygen defgroup, and importantly, not the subgroups of this defgroup.

If you define a defgroup with defgroup children, you need to tell Sphinx
to display API information about the child defgroups too.  (If the
parent defgroup has no entities of it's own, you can leave that out.)

This patch replaces the (empty) display of the parent defgroup with that
of its two child defgroups.

It also fixes an unnecessarily long text line while I was in there.

Fixes: #7666

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>