Clarify delete_image description

This also runs when an image is removed explicitly, not just when an entire
release is merged or deleted.

README.md

@@ -123,7 +123,7 @@ Event types and their expected `message` format are documented below.
 | ------------- | ----------------------------------------------------------------------- | ----------------------------------------------------------------------- |
 | index         | `{"gid": UUID}`                                                         | uploads index.json and MB metadata to the IA                            |
 | copy_image    | `{"artwork_id": INT, "old_gid": UUID, "new_gid": UUID, "suffix": TEXT}` | copies an image from one bucket to another (after a release is merged)  |
-| delete_image  | `{"gid": UUID, "artwork_id": INT, "suffix": TEXT}`                      | deletes an image (after a release is merged or deleted)                 |
+| delete_image  | `{"gid": UUID, "artwork_id": INT, "suffix": TEXT}`                      | deletes an image (including after a release is merged or deleted)       |
 | deindex       | `{"gid": UUID}`                                                         | deletes index.json (after a release is deleted)                         |
 | noop          | `{}` or `{"fail": BOOL}`                                                | for debugging (does nothing, or fails if `{"fail": true}` is specified) |