Edited doc for draw_tile_grid

- Yes! They are super helpful for when you look at your old code and
completely forget how it works.

tile_grid.py

@@ -10,7 +10,8 @@ from tile import fly
 def draw_tile_grid(width, height, tile_size, tile_function):
     """Draws a (width x height) grid, with tile_function drawn on each tile.
 
-    (Your explanation here.)
+    This function draws a grid of tiles row by row, filling each row in
+    left to right. 
     """
     for y in range(height):
         for x in range(width):