docs: document TurtleGraphicsError

Author: openclaw

Doc/library/turtle.rst

@@ -54,6 +54,11 @@ direction it is facing, drawing a line as it moves.  Give it the command
 In Python, turtle graphics provides a representation of a physical "turtle"
 (a little robot with a pen) that draws on a sheet of paper on the floor.
 
+.. exception:: TurtleGraphicsError
+
+   Exception raised when an invalid turtle graphics operation is requested,
+   such as using an unsupported shape or an invalid color value.
+
 It's an effective and well-proven way for learners to encounter
 programming concepts and interaction with software, as it provides instant,
 visible feedback. It also provides convenient access to graphical output