ACPI: property: graph: Improve graph documentation for port/ep numbering

Document that if a port has a single endpoint only, its value shall be
zero. Similarly, if a device object only has a single port, its value
shlla be zero.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>