Fix #565

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

docs/examples/mock_demo.py

@@ -0,0 +1,28 @@
+from gpiozero.pins.mock import MockFactory
+from gpiozero import Device, Button, LED
+from time import sleep
+
+# Set the default pin factory to a mock factory
+Device.pin_factory = MockFactory()
+
+# Construct a couple of devices attached to mock pins 16 and 17, and link the
+# devices
+led = LED(17)
+btn = Button(16)
+led.source = btn.values
+
+# Here the button isn't "pushed" so the LED's value should be False
+print(led.value)
+
+# Get a reference to mock pin 16 (used by the button)
+btn_pin = Device.pin_factory.pin(16)
+
+# Drive the pin low (this is what would happen eletrically when the button is
+# pushed)
+btn_pin.drive_low()
+sleep(0.1) # give source some time to re-read the button state
+print(led.value)
+
+btn_pin.drive_high()
+sleep(0.1)
+print(led.value)