Just ran into this today after finding out about the game manuals feature.
Its definitely a bug or an oversight. The Bulk Edit wizard is simply taking the field name and asking for what it should be set to, and Game Manual Path expects a path to a single file, not a directory. This is probably because the Game Manual Path field doesnt do any sort of lookup/matching, it relies on you supplying the file explicitly.
It would be great to have an update for this so we can give a directory per-platform with all of the manuals, assuming they are named the same - I'm okay with manually fixing anything that's not close and can't be matched.