When a rollbacker hides or unhides a post (or when an oversighter deletes or undeletes a post), they should have the option to add a note to explain why they did it.
The note would appear as a text box overlay, to be shown above the 'Hide this post' button, on the right of the feedback page, with a cartoon tip pointing to it. This small 3-line text box overlay would be about 30% of total width, and include a headline above the text box and button below the text box saying:
'Add a note [bold title]
[ text box with this text prompt:
'Please explain why you are deleting this post.'
. . . . . . . . . . . . . . ]'
[ Post your note]' (smaller version of the blue 'Post your feedback' button.)
This feature is related to the Activity feature, which will take a few weeks to implement, but I am noting it here for the record.