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

src/backend/commands/copy.c

@@ -2604,6 +2604,9 @@ CopyMultiInsertInfoCleanup(CopyMultiInsertInfo *miinfo)
  * Get the next TupleTableSlot that the next tuple should be stored in.
  *
  * Callers must ensure that the buffer is not full.
+ *
+ * Note: 'miinfo' is unused but has been included for consistency with the
+ * other functions in this area.
  */
 static inline TupleTableSlot *
 CopyMultiInsertInfoNextFreeSlot(CopyMultiInsertInfo *miinfo,