Mention that keep_files plugin keeps derivatives as well

Closes #553

doc/plugins/keep_files.md

@@ -2,10 +2,11 @@
 title: Keep Files
 ---
 
-The [`keep_files`][keep_files] plugin prevents file deletion when the attacher
-is about to destroy currently attached or previously attached file. This
-functionality is useful when implementing soft deletes, versioning, or in
-general any scenario where you need to track history.
+The [`keep_files`][keep_files] plugin prevents the attached file (and any of
+its [derivatives]) from being deleted when the attachment would normally be
+destroyed, which happens when the attachment is removed/replaced, or when the
+record is deleted. This functionality is useful when implementing soft deletes,
+versioning, or in general any scenario where you need to keep history.
 
 ```rb
 plugin :keep_files
@@ -17,3 +18,4 @@ photo.image.exists? #=> true
 ```
 
 [keep_files]: https://github.com/shrinerb/shrine/blob/master/lib/shrine/plugins/keep_files.rb
+[derivatives]: https://shrinerb.com/docs/plugins/derivatives