machine: add PinOutputOpenDrain and PinDisable

These two are used on the Teensy boards (teensy36, teensy40, teensy41)
and seem quite sensible to me, but weren't documented before. It is my
goal to let the public API of the machine package match the
documentation here, so that's why I'd like to add these two pin modes.

Author: aykevl

content/docs/reference/machine.md

@@ -24,10 +24,12 @@ Boards have various pins defined as constants. For example, ATsamd21 based chips
 type PinMode uint8
 
 const (
-    PinOutput        PinMode = ...
-    PinInput         PinMode = ...
-    PinInputPullup   PinMode = ...
-    PinInputPulldown PinMode = ... // not always available
+    PinOutput          PinMode = ...
+    PinInput           PinMode = ...
+    PinInputPullup     PinMode = ...
+    PinInputPulldown   PinMode = ... // not always available
+    PinOutputOpenDrain PinMode = ... // not always available
+    PinDisable         PinMode = ... // not always available
 )
 ```