Fix #3923, add instructions for taking down a shot given an image URL

docs/takedown.md

@@ -23,3 +23,19 @@ WHERE id = '29f8d8asd93/example.com'
 In the future we might identify other categories of blocked content, but currently the only kind is `'dmca'`.
 
 For a non-owner the link should now return a 404; for the owner the link will give a notification.
+
+### Getting a shot ID from an image URL
+
+You may get a report for a URL, instead of an image. The image ID is a UUID *with* extension. So for instance, for a URL `https://screenshots.firefoxusercontent.com/images/36333d2b-f158-468d-8113-38f18c3ede9a.png` the ID is `36333d2b-f158-468d-8113-38f18c3ede9a.png`
+
+To get the shot ID, execute:
+
+```sql
+SELECT shotid FROM images WHERE id = '36333d2b-f158-468d-8113-38f18c3ede9a.png'
+```
+
+If the image row had been deleted for some ways, it is possible to find the accompanying shot, but requires a full table scan (this may be very slow!):
+
+```sql
+SELECT id FROM data WHERE value LIKE '%36333d2b-f158-468d-8113-38f18c3ede9a.png%';
+```