Add documentation and NotImplementedError to abstract classes in abstract.py.

src/abstract.py

-"""Collection of abstract classes for type hinting in the rest of the project."""
+"""Collection of abstract classes for type hinting in the rest of the project.
+
+(For students: don't worry about the code that's in here.
+These classes are just providing examples of how to use the proximity sensor,
+the LED controller, and the touch sensor for type hinting.
+The actual implementations of the classes are on the device.
+You'll learn more about abstract classes in CS2.
+"""
 # We aren't using the abc module because it isn't available on circuitpy.
 
 class ProximitySensor:
@@ -6,14 +13,30 @@ class ProximitySensor:
 
     @property
     def proximity(self) -> int:
-        return -1
+        """Integer in [0, 255] indicating proximity of object to sensor.
+
+        Lower is farther away and higher is closer.
+
+        Example:
+            `apds.proximity`
+        """
+        raise NotImplementedError
 
 
 class LEDController:
     """Abstract LED controller."""
 
     def fill(self, color: tuple[int, int, int]) -> None:
-        pass
+        """Change the color of the LED being controlled.
+
+        Args:
+            color: Three-tuple of integers in [0, 255] representing the values
+                of red, green, and blue to display on the LED.
+
+        Example:
+            `px.fill((0, 0, 0))` turns the LED off
+        """
+        raise NotImplementedError
 
 
 class TouchSensor:
@@ -21,4 +44,9 @@ class TouchSensor:
 
     @property
     def value(self) -> bool:
-        return False
+        """If the sensor is being touched.
+
+        Example:
+            `touch.value`
+        """
+        raise NotImplementedError