Add comment to explain an unused function parameter

 Add comment to explain an unused function parameter
 
 Removing the unused 'miinfo' parameter has been raised a couple of times
 now.  It was decided in the 2nd discussion below that we're going to leave
 it alone.  It seems like it might be useful to add a comment to mention
 this fact so that nobody wastes any time in the future proposing its
 removal again.
 
 Discussion: https://postgr.es/m/CAApHDvpCf-qR5HC1rXskUM4ToV+3YDb4-n1meY=vpAHsRS_1PA@mail.gmail.com
 Discussion: https://postgr.es/m/CAE9k0P%3DFvcDswnSVtRpSyZMpcAWC%3DGp%3DifZ0HdfPaRQ%3D__LBtw%40mail.gmail.com