pgsql: Minor code cleanup in asynchronous execution support.

Minor code cleanup in asynchronous execution support.

This is cleanup for commit 27e1f1456:

* ExecAppendAsyncEventWait(), which was modified a bit further by commit
  a8af856d3, duplicated the same nevents calculation.  Simplify the code
  a little bit to avoid the duplication.  Update comments there.
* Add an assertion to ExecAppendAsyncRequest().
* Update a comment about merging the async_capable options from input
  relations in merge_fdw_options(), per complaint from Kyotaro Horiguchi.
* Add a comment for fetch_more_data_begin().

Author: Etsuro Fujita
Discussion: https://postgr.es/m/CAPmGK1637W30Wx3MnrReewhafn6F_0J76mrJGoFXFnpPq4QfvA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bb684c82f73316b88f9bcb321128a4347b5206fe

Modified Files
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contrib/postgres_fdw/postgres_fdw.c |  8 +++++++-
src/backend/executor/nodeAppend.c   | 18 +++++++++++++-----
2 files changed, 20 insertions(+), 6 deletions(-)