pgsql: Add explanatory comment to prune_freeze_setup()

Add explanatory comment to prune_freeze_setup()

heap_page_prune_and_freeze() fills in PruneState->deadoffsets, the array
of OffsetNumbers of dead tuples. It is returned to the caller in the
PruneFreezeResult. To avoid having two copies of the array, the
PruneState saves only a pointer to the array. This was a bit unusual and
confusing, so add a clarifying comment.

Author: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Suggested-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAEoWx2=jiD1nqch4JQN+odAxZSD7mRvdoHUGJYN2r6tQG_66yQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bfe5c4bec75091d5c91813ff9c6994a169ceb8ef

Modified Files
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src/backend/access/heap/pruneheap.c | 7 +++++++
1 file changed, 7 insertions(+)