pins: Document support for '~' to enable pull-down resistors

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

config/example.cfg

@@ -11,11 +11,13 @@
 # A note on pin names: pins may be configured with a hardware name
 # (such as "PA4") or with an Arduino alias name (such as "ar29" or
 # "analog3"). In order to use Arduino names, the pin_map variable in
-# the mcu section must be present and have a value of "arduino".
-# Pin names may be preceded by an '!' to indicate that a reverse
-# polarity should be used (eg, trigger on low instead of high). Input
-# pins may be preceded by a '^' to indicate that a hardware pull-up
-# resistor should be enabled for the pin.
+# the mcu section must be present and have a value of "arduino". Pin
+# names may be preceded by an '!' to indicate that a reverse polarity
+# should be used (eg, trigger on low instead of high). Input pins may
+# be preceded by a '^' to indicate that a hardware pull-up resistor
+# should be enabled for the pin. If the micro-controller supports
+# pull-down resistors then an input pin may alternatively be preceded
+# by a '~'.
 
 
 # The stepper_x section is used to describe the stepper controlling

klippy/pins.py

@@ -183,7 +183,7 @@ class PrinterPins:
         if [c for c in '^~!: ' if c in pin]:
             format = ""
             if can_pullup:
-                format += "[^] "
+                format += "[^~] "
             if can_invert:
                 format += "[!] "
             raise error("Invalid pin description '%s'\n"