add some explanation of summary stats outputs

Author: drbenvincent

docs/source/notebooks/staggered_did_pymc.ipynb

@@ -452,7 +452,9 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## View Summary Statistics\n"
+    "## View Summary Statistics\n",
+    "\n",
+    "The `n_obs` column in the event-time ATT table shows the number of treated unit-time observations contributing to each event-time estimate. This varies across event-times because different cohorts have different lengths of post-treatment history. For example, units treated early in the panel contribute observations at all event-times, while units treated later only contribute to earlier event-times (e.g., event-time 0, 1, 2) before the panel ends.\n"
    ]
   },
   {