Fix #565

Add mock pins docs and tidy up some other bits of the pins docs

gpiozero/pins/rpigpio.py

@@ -58,6 +58,12 @@ class RPiGPIOFactory(LocalPiFactory):
 
 
 class RPiGPIOPin(LocalPiPin):
+    """
+    Pin implementation for the `RPi.GPIO`_ library. See :class:`RPiGPIOFactory`
+    for more information.
+
+    .. _RPi.GPIO: https://pypi.python.org/pypi/RPi.GPIO
+    """
     GPIO_FUNCTIONS = {
         'input':   GPIO.IN,
         'output':  GPIO.OUT,