Add a docstring

src/sage_docbuild/conf.py

@@ -919,9 +919,17 @@ class will be properly documented inside its surrounding class.
     return skip
 
 def clean_jupyter_execute(app, exception):
-    jupyter_execute_path = os.path.join(app.outdir, 'jupyter_execute')
-    if os.path.exists(jupyter_execute_path):
-        shutil.rmtree(jupyter_execute_path)
+    """
+    If there was no exception during the doc build, we remove the useless
+    ``jupyter_execute`` directory, which would only bloat the doc tree.
+
+    If there was an exception, the ``jupyter_execute`` directory may be useful
+    for debugging and hence we don't remove it.
+    """
+    if exception is None:  # only proceed if there were no build errors
+        jupyter_execute_path = os.path.join(app.outdir, 'jupyter_execute')
+        if os.path.exists(jupyter_execute_path):
+            shutil.rmtree(jupyter_execute_path)
 
 
 class SagecodeTransform(SphinxTransform):