[libclang/python][NFC] Document how null cursors are handled (#140499)

Now that we looked at this aspect closely and fixed things, I think we
can document this.

Author: Endilll

clang/bindings/python/clang/cindex.py

@@ -1585,6 +1585,8 @@ class Cursor(Structure):
     """
     The Cursor class represents a reference to an element within the AST. It
     acts as a kind of iterator.
+
+    Null cursors are mapped to None.
     """
 
     _fields_ = [("_kind_id", c_int), ("xdata", c_int), ("data", c_void_p * 3)]