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