serial: add pci-serial documentation

Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

docs/specs/pci-serial.txt

+
+QEMU pci serial devices
+=======================
+
+There is one single-port variant and two muliport-variants.  Linux
+guests out-of-the box with all cards.  There is a Windows inf file
+(docs/qemupciserial.inf) to setup the single-port card in Windows
+guests.
+
+
+single-port card
+----------------
+
+Name:   pci-serial
+PCI ID: 1b36:0002
+
+PCI Region 0:
+   IO bar, 8 bytes long, with the 16550 uart mapped to it.
+   Interrupt is wired to pin A.
+
+
+multiport cards
+---------------
+
+Name:   pci-serial-2x
+PCI ID: 1b36:0003
+
+Name:   pci-serial-4x
+PCI ID: 1b36:0004
+
+PCI Region 0:
+   IO bar, with two/four 16550 uart mapped after each other.
+   The first is at offset 0, second at offset 8, ...
+   Interrupt is wired to pin A.

hw/serial-pci.c

@@ -23,6 +23,8 @@
  * THE SOFTWARE.
  */
 
+/* see docs/specs/pci-serial.txt */
+
 #include "serial.h"
 #include "pci.h"