toString() methods should not be used as displayers. They shouldn't be substitutes for displayers. They should be used for printing logs/exceptions.
FTR this code is used to display attachment diff in the diff UI and in the watchlist HTML email.
Added in rev 62368e9423925fbd496a5cc057a076b37b62f9ee with message "[misc] Improve javadoc, add AttachmentDiff.toString()"