DOC: Add explanatory comment

Co-authored-by: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>

Author: tacaswell

tools/github_stats.py

@@ -207,7 +207,7 @@ def report(issues, show_urls=False):
             since -= td
 
     since = round_hour(since)
-    end_date = datetime.today()
+    end_date = datetime.today()  # will be overwritten for closed milestones
 
     milestone = opts.milestone
     project = opts.project