Documentation: net: phy: improve explanation when to specify the PHY ID

The old description basically read like "ethernet-phy-idAAAA.BBBB" can
be specified when you know the actual PHY ID. However, specifying this
has a side-effect: it forces Linux to bind to a certain PHY driver (the
one that matches the ID given in the compatible string), ignoring the ID
which is reported by the actual PHY.
Whenever a device is shipped with (multiple) different PHYs during it's
production lifetime then explicitly specifying
"ethernet-phy-idAAAA.BBBB" could break certain revisions of that device.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/phy.txt

@@ -19,8 +19,9 @@ Optional Properties:
   specifications. If neither of these are specified, the default is to
   assume clause 22.
 
-  If the phy's identifier is known then the list may contain an entry
-  of the form: "ethernet-phy-idAAAA.BBBB" where
+  If the PHY reports an incorrect ID (or none at all) then the
+  "compatible" list may contain an entry with the correct PHY ID in the
+  form: "ethernet-phy-idAAAA.BBBB" where
      AAAA - The value of the 16 bit Phy Identifier 1 register as
             4 hex digits. This is the chip vendor OUI bits 3:18
      BBBB - The value of the 16 bit Phy Identifier 2 register as