ACPI: property: Add code comments explaining what is going on

ACPI: property: Add code comments explaining what is going on

[ Upstream commit 737c3a09dcf69ba2814f3674947ccaec1861c985 ]

In some places in the ACPI device properties handling code, it is
unclear why the code is what it is.  Some assumptions are not documented
and some pieces of code are based on knowledge that is not mentioned
anywhere.

Add code comments explaining these things.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Tested-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Stable-dep-of: baf60d5cb8bc ("ACPI: property: Do not pass NULL handles to acpi_attach_data()")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>