Document the rules for DMI matching with PCI subsystem IDs

The rules may change in the future, but right now it is important that
the comments match the code.

Corresponding to flashrom svn r893.
Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>

board_enable.c

@@ -1160,9 +1160,10 @@ static int board_asus_a7v600x(const char *name)
  * subsystem/card IDs in a sane manner.
  *
  * Keep the second set NULLed if it should be ignored. Keep the subsystem IDs
- * NULLed if they don't identify the board fully. But please take care to
- * provide an as complete set of pci ids as possible; autodetection is the
- * preferred behaviour and we would like to make sure that matches are unique.
+ * NULLed if they don't identify the board fully and if you can't use DMI.
+ * But please take care to provide an as complete set of pci ids as possible;
+ * autodetection is the preferred behaviour and we would like to make sure that
+ * matches are unique.
  *
  * If PCI IDs are not sufficient for board matching, the match can be further
  * constrained by a string that has to be present in the DMI database for
@@ -1171,6 +1172,11 @@ static int board_asus_a7v600x(const char *name)
  * or the end (with a $ at the end). Both anchors may be specified at the
  * same time to match the full field.
  *
+ * When a board is matched through DMI, the first and second main PCI IDs
+ * and the first subsystem PCI ID have to match as well. If you specify the
+ * first subsystem ID as 0x0:0x0, the DMI matching code expects that the
+ * subsystem ID of that device is indeed zero.
+ *
  * The coreboot ids are used two fold. When running with a coreboot firmware,
  * the ids uniquely matches the coreboot board identification string. When a
  * legacy bios is installed and when autodetection is not possible, these ids