pgsql: Improve comments for ExecAppendAsyncEventWait.

Improve comments for ExecAppendAsyncEventWait.

We previously commented inside this function about when to wait and when
to poll; to improve readability, move this explanation to the top of the
function with some modifications, and replace the old comment with a
brief note.

Author: Chao Li <lic(at)highgo(dot)com>
Reviewed-by: Etsuro Fujita <etsuro(dot)fujita(at)gmail(dot)com>
Discussion: https://postgr.es/m/C05FA422-F3E6-4198-AB7F-70570C157001%40gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/58aecdd064c5f156e04e96c3fa4074f4ae3e19ca

Modified Files
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src/backend/executor/nodeAppend.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)