docs: further explain ButtonPacket.button property

Explain that this attribute corresponds to whatever the user has pressed
or released.

Author: ktdreyer

adafruit_bluefruit_connect/button_packet.py

@@ -101,7 +101,8 @@ def to_bytes(self):
 
     @property
     def button(self):
-        """A single character string (not bytes) specifying the button."""
+        """A single character string (not bytes) specifying the button that
+        the user pressed or released."""
         return self._button
 
     @property