sysfs: Document new "group visible" helpers

sysfs: Document new "group visible" helpers

Add documentation and examples for how to use
DEFINE_SYSFS_GROUP_VISIBLE() and SYSFS_GROUP_VISIBLE(). Recall that the
motivation for this work is that it is easier to reason about the
lifetime of statically defined sysfs attributes that become visible at
device_add() time rather than dynamically adding them later.
DEFINE_SYSFS_GROUP_VISIBLE() tackles one of the reasons to opt for
dynamically created attributes which did not have a facility for hiding
empty directories.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/170863446065.1479840.10697164014098377292.stgit@dwillia2-xfh.jf.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>