Add commentary explaining why MaxIndexTuplesPerPage calculation is safe.

Add commentary explaining why MaxIndexTuplesPerPage calculation is safe.

MaxIndexTuplesPerPage ignores the fact that btree indexes sometimes
store tuples with no data payload.  But it also ignores the possibility
of "special space" on index pages, which offsets that, so that the
result isn't an underestimate.  This all seems worth documenting, though.

In passing, remove #define MinIndexTupleSize, which was added by
commit 2c03216d8 but not used in that commit nor later ones.

Comment text by me; issue noticed by Peter Geoghegan.

Discussion: https://postgr.es/m/CAH2-WzkQmb54Kbx-YHXstRKXcNc+_87jwV3DRb54xcybLR7Oig@mail.gmail.com