pgsql: Improve comments for trivial_subqueryscan().

Improve comments for trivial_subqueryscan().

This function can be called from mark_async_capable_plan(), a helper
function for create_append_plan(), before set_subqueryscan_references(),
to determine the triviality of a SubqueryScan that is a child of an
Append plan node, which is done before doing finalize_plan() on the
SubqueryScan (if necessary) and set_plan_references() on the subplan,
unlike when called from set_subqueryscan_references().  The reason why
this is safe wouldn't be that obvious, so add comments explaining this.

Follow-up for commit c2bb02bc2.

Reviewed by Zhihong Yu.

Discussion: https://postgr.es/m/CAPmGK17%2BGiJBthC6va7%2B9n6t75e-M1N0U18YB2G1B%2BE5OdrNTA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4a8a5dd7f59cd8dd9969cef1969cb5f7c5124eec

Modified Files
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src/backend/optimizer/plan/setrefs.c | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)