CI: treat documentation warnings as errors

We do not want to merge documentation that produces Sphinx warnings.

scripts/kernel-doc uses environment variable KDOC_WERROR to determine
if warnings should be treated as errors.

Reported-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/.azure-pipelines.yml b/.azure-pipelines.yml
index 76ffdee..3c1846a 100644
--- a/.azure-pipelines.yml
+++ b/.azure-pipelines.yml
@@ -89,7 +89,7 @@
           virtualenv -p /usr/bin/python3 /tmp/venvhtml
           . /tmp/venvhtml/bin/activate
           pip install -r doc/sphinx/requirements.txt
-          make htmldocs
+          make htmldocs KDOC_WERROR=1
           make infodocs
 
   - job: todo