docs: explain ButtonPacket.pressed property values

As alluded in the __init__() docstring, explain that the "pressed"
property is False when a button is released.

Author: ktdreyer

adafruit_bluefruit_connect/button_packet.py

@@ -105,7 +105,7 @@ def button(self):
 
     @property
     def pressed(self):
-        """True if button is pressed."""
+        """``True`` if button is pressed, or ``False`` if it is released."""
         return self._pressed