add documentation

doc/usage/extensions/autosectionlabel.rst

@@ -45,3 +45,11 @@ Configuration
    example, when set 1 to ``autosectionlabel_maxdepth``, labels are generated
    only for top level sections, and deeper sections are not labeled.  It
    defaults to ``None`` (disabled).
+
+
+Debugging
+---------
+The ``WARNING: undefined label`` indicates that your reference in
+:rst:role:`ref` is mis-spelled. Invoking :program:`sphinx-build` with ``-vv``
+(see :option:`-v`) will print all section names and the labels that have been
+generated for them. This output can help finding the right reference label.