fsdax: improve the FS_DAX Kconfig description and help text

Rename the main option text to clarify it is for file system access,
and add a bit of text that explains how to actually switch a nvdimm
to a fsdax capable state.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210826135510.6293-2-hch@lst.de


Signed-off-by: Dan Williams <dan.j.williams@intel.com>