Update mro method docstring with additional possible exceptions

Prevents users from having to dig through the call chain to figure out which possible
exceptions are raised

astroid/scoped_nodes.py

@@ -2632,6 +2632,8 @@ class ClassDef(mixins.FilterStmtsMixin, LocalsDictNodeNG,
         :raises NotImplementedError: If this is an old style class,
             since they don't have the concept of an MRO.
+        :raises DuplicateBasesError: Duplicate bases in the same class base
+        :raises InconsistentMroError: A class' MRO is inconsistent
         """
         if not self.newstyle: