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

[Jonathan Corbet <corbet@xxxxxxx>]

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 when combined with certain versions of Sphinx.

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

Reported-by: Akira Yokosawa <akiyks@xxxxxxxxx>
Fixes: d6d1df92c25f ("docs: automarkup: Mark up undocumented entities too")
Signed-off-by: Jonathan Corbet <corbet@xxxxxxx>
---
TODO for the future: figure out where the warning is going

 Documentation/sphinx/automarkup.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Documentation/sphinx/automarkup.py b/Documentation/sphinx/automarkup.py
index e67eb8e19c22..563033f764bb 100644
--- a/Documentation/sphinx/automarkup.py
+++ b/Documentation/sphinx/automarkup.py
@@ -240,7 +240,7 @@ def add_and_resolve_xref(app, docname, domain, reftype, target, contnode=None):
     # mark it as a broken xref
     #
     if contnode:
-        contnode.set_class("broken_xref")
+        contnode['classes'].append("broken_xref")
     return contnode
 
 #
-- 
2.49.0