This has been discussed previously in T12723: Give users the ability to edit their own edit summaries

A summary can somehow be incomplete, and it should be possible to make it more complete or accurate. One possibility could be to append a new summary, and overstrike (delete) the previous summary. In the special case where there are no previous summary it would only be shown as an additional entry.

I'm pretty sure only the author of the revision should be able to add to the summary.