docs: sphinx: avoid using the deprecated node.set_class()

Docutils emits a deprecation warning when the set_class() element method is
used; that warning disappears into the ether, but it also causes a crash
with docutils 0.19.

Avoid the deprecated function and just append directly to the "classes"
attribute like the documentation says instead.

Reported-by: Akira Yokosawa <[e-mail]>
Tested-by: Akira Yokosawa <[e-mail]>
Closes: https://lore.kernel.org/[e-mail]/
Fixes: d6d1df92c25f ("docs: automarkup: Mark up undocumented entities too")
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/sphinx/automarkup.py

···
     # mark it as a broken xref
     #
     if contnode:
-        contnode.set_class("broken_xref")
+        contnode['classes'].append("broken_xref")
     return contnode
 
 #