add reference to the PR

Author: dcoudert

src/sage/graphs/generic_graph.py

@@ -3580,7 +3580,7 @@ def multiple_edges(self, to_undirected=False, labels=True, sort=False, key=None)
             [(1, 2, 'h'), (2, 1, 'g')]
 
         Using the ``key`` argument to order multiple edges of incomparable
-        types::
+        types (see :trac:`35903`)::
 
             sage: G = Graph([('A', 'B', 3), (1, 2, 1), ('A', 'B', 4), (1, 2, 2)], multiedges=True)
             sage: G.multiple_edges(sort=True)
@@ -12619,7 +12619,7 @@ def edge_boundary(self, vertices1, vertices2=None, labels=True, sort=False, key=
             [(0, 1), (0, 2)]
 
         Using the ``key`` argument to order multiple edges of incomparable
-        types::
+        types (see :trac:`35903`)::
 
             sage: G = Graph([(1, 'A', 4), (1, 2, 3)])
             sage: G.edge_boundary([1], sort=True)

src/sage/graphs/graph.py

@@ -1543,7 +1543,7 @@ def is_tree(self, certificate=False, output='vertex'):
             sage: G.is_tree(certificate=True, output='edge')
             (False, [(0, 0, None)])
 
-        Case of edges with incomparable types::
+        Case of edges with incomparable types (see :trac:`35903`)::
 
             sage: G = Graph(multiedges=True)
             sage: G.add_cycle(['A', 1, 2, 3])