pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/101f903e51f52bf595cd8177d2e0bc6fe9000762

Modified Files
--------------
src/backend/commands/copy.c | 3 +++
1 file changed, 3 insertions(+)