-Left-click any edge to toggle it on or off, or click and drag to draw
-an entire rectangle (or line) on the grid in one go (removing any
-existing edges within that rectangle).
+Alternatively, use the cursor keys to move the position indicator
+around the board. Pressing the return key then allows you to use the
+cursor keys to drag a rectangle out from that position, and pressing
+the return key again completes the rectangle. Using the space bar
+instead of the return key allows you to erase the contents of a
+rectangle without affecting its edges, as above.