ABI: sysfs-bus-pci: add documentation for modalias

Even being available since 2005, there's no documentation for
modalias.
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9ceb1fcdbef3c0d2d0368dcc2f19084a3e529ad1.1632750608.git.mchehab+huawei@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-bus-pci

@@ -187,6 +187,24 @@ Description:
 		The symbolic link points to the PCI device sysfs entry of the
 		Physical Function this device associates with.
 
+What:		/sys/bus/pci/devices/.../modalias
+Date:		May 2005
+Contact:	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+Description:
+		This attribute indicates the PCI ID of the device object.
+
+		That is in the format:
+		pci:vXXXXXXXXdXXXXXXXXsvXXXXXXXXsdXXXXXXXXbcXXscXXiXX,
+		where:
+
+		    - vXXXXXXXX contains the vendor ID;
+		    - dXXXXXXXX contains the device ID;
+		    - svXXXXXXXX contains the sub-vendor ID;
+		    - sdXXXXXXXX contains the subsystem device ID;
+		    - bcXX contains the device class;
+		    - scXX contains the device subclass;
+		    - iXX contains the device class programming interface.
+
 What:		/sys/bus/pci/slots/.../module
 Date:		June 2009
 Contact:	linux-pci@vger.kernel.org