doc: Cope with Sphinx logging deprecations

Recent versions of sphinx will emit messages like:

  Documentation/sphinx/kerneldoc.py:103:
     RemovedInSphinx20Warning: app.warning() is now deprecated.
     Use sphinx.util.logging instead.

Switch to sphinx.util.logging to make this unsightly message go away.
Alas, that interface was only added in version 1.6, so we have to add a
version check to keep things working with older sphinxes.

Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sphinx/kernellog.py

@@ -0,0 +1,28 @@
+# SPDX-License-Identifier: GPL-2.0
+#
+# Sphinx has deprecated its older logging interface, but the replacement
+# only goes back to 1.6.  So here's a wrapper layer to keep around for
+# as long as we support 1.4.
+#
+import sphinx
+
+if sphinx.__version__[:3] >= '1.6':
+    UseLogging = True
+    from sphinx.util import logging
+    logger = logging.getLogger('kerneldoc')
+else:
+    UseLogging = False
+
+def warn(app, message):
+    if UseLogging:
+        logger.warning(message)
+    else:
+        app.warn(message)
+
+def verbose(app, message):
+    if UseLogging:
+        logger.verbose(message)
+    else:
+        app.verbose(message)
+
+