pci: Update documentation to make 'compatible' string optional

Reword the documentation to make it clear the compatible string is now
optional, yet still matching on it takes precedence over PCI IDs and
PCI classes.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

doc/driver-model/pci-info.txt

@@ -34,11 +34,15 @@ under that bus.
 Note that this is all done on a lazy basis, as needed, so until something is
 touched on PCI (eg: a call to pci_find_devices()) it will not be probed.
 
-PCI devices can appear in the flattened device tree. If they do this serves to
-specify the driver to use for the device. In this case they will be bound at
-first. Each PCI device node must have a compatible string list as well as a
-<reg> property, as defined by the IEEE Std 1275-1994 PCI bus binding document
-v2.1. Note we must describe PCI devices with the same bus hierarchy as the
+PCI devices can appear in the flattened device tree. If they do, their node
+often contains extra information which cannot be derived from the PCI IDs or
+PCI class of the device. Each PCI device node must have a <reg> property, as
+defined by the IEEE Std 1275-1994 PCI bus binding document v2.1. Compatible
+string list is optional and generally not needed, since PCI is discoverable
+bus, albeit there are justified exceptions. If the compatible string is
+present, matching on it takes precedence over PCI IDs and PCI classes.
+
+Note we must describe PCI devices with the same bus hierarchy as the
 hardware, otherwise driver model cannot detect the correct parent/children
 relationship during PCI bus enumeration thus PCI devices won't be bound to
 their drivers accordingly. A working example like below: