docs/library/machine.Pin: Update to use preferred DRIVE_x constants.

Update documents with new common names for the drive strength constants.

docs/library/machine.Pin.rst

@@ -87,9 +87,9 @@ Constructors
        output pin value if given, otherwise the state of the pin peripheral remains
        unchanged.
 
-     - ``drive`` specifies the output power of the pin and can be one of: ``Pin.LOW_POWER``,
-       ``Pin.MED_POWER`` or ``Pin.HIGH_POWER``.  The actual current driving capabilities
-       are port dependent.  Not all ports implement this argument.
+     - ``drive`` specifies the output power of the pin and can be one of: ``Pin.DRIVE_0``,
+       ``Pin.DRIVE_1``, etc., increasing in drive strength.  The actual current driving
+       capabilities are port dependent.  Not all ports implement this argument.
 
      - ``alt`` specifies an alternate function for the pin and the values it can take are
        port dependent.  This argument is valid only for ``Pin.ALT`` and ``Pin.ALT_OPEN_DRAIN``
@@ -260,11 +260,13 @@ not all constants are available on all ports.
    Selects whether there is a pull up/down resistor.  Use the value
    ``None`` for no pull.
 
-.. data:: Pin.LOW_POWER
-          Pin.MED_POWER
-          Pin.HIGH_POWER
+.. data:: Pin.DRIVE_0
+          Pin.DRIVE_1
+          Pin.DRIVE_2
 
-   Selects the pin drive strength.
+   Selects the pin drive strength.  A port may define additional drive
+   constants with increasing number corresponding to increasing drive
+   strength.
 
 .. data:: Pin.IRQ_FALLING
           Pin.IRQ_RISING