I recently merged a person and used one of the default reason statements, and forgot to add the most important part "I believe the census takers hand writing was a little hard to interpret and his age was recorded incorrectly." To bad I cannot add that to the merge now.
There are two ways of addressing this. Firstly, you could restore the merged ID and start over again - allowing you now to add your statement. Otherwise, add a Note in the Collaboration section of the surviving ID, explaining your justification for making the merge, with the words you originally intended to use. Either way, this will find its way into (at least) the Change Log of the surviving record.
I haven't done this myself, but have sometimes detached a source when I have found I have not provided a reason statement, then immediately reattached it (so it includes my intended statement).