Fix misleading comment in tuplesort.c

Fix misleading comment in tuplesort.c

A comment in tuplesort.c was claiming that the code was defining
INITIAL_MEMTUPSIZE so that it *does not* exceed
ALLOCSET_SEPARATE_THRESHOLD, but the code actually ensures that we
purposefully *do* exceed ALLOCSET_SEPARATE_THRESHOLD for the initial
allocation of the tuples array, as per reasons detailed in the
commentary of grow_memtuples().

Also, there's not much need to repeat the mention about
ALLOCSET_SEPARATE_THRESHOLD in each location where INITIAL_MEMTUPSIZE is
used, so remove those comments.

Author: ChangAo Chen <cca5507@qq.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Discussion: https://postgr.es/m/tencent_6FA14F85D6B5B5291532D6789E07F4765C08%40qq.com