Update sourcevalues docs as suggested in lurch's review

docs/examples/led_button_loop.py

@@ -1,7 +1,9 @@
 from gpiozero import LED, Button
+from time import sleep
 
 led = LED(17)
 button = Button(2)
 
 while True:
     led.value = button.value
+    sleep(0.01)

docs/examples/negated.py

@@ -4,6 +4,7 @@ from signal import pause
 
 led = LED(4)
 btn = Button(17)
+
 led.source = negated(btn.values)
 
 pause()

docs/examples/random_values.py

@@ -4,5 +4,6 @@ from signal import pause
 
 led = PWMLED(4)
 led.source = random_values()
+led.source_delay = 0.1
 
 pause()

docs/examples/source_value_processing.py

@@ -0,0 +1,13 @@
+from gpiozero import Button, LED
+from signal import pause
+
+def opposite(values):
+    for value in values:
+        yield not value
+
+led = LED(4)
+btn = Button(17)
+
+led.source = opposite(btn.values)
+
+pause()