pgsql: aio: Fix assertion, clarify README

aio: Fix assertion, clarify README

The assertion wouldn't have triggered for a long while yet, but this won't
accidentally fail to detect the issue if/when it occurs.

Author: Matthias van de Meent <boekewurm+postgres@gmail.com>
Discussion: https://postgr.es/m/CAEze2Wj-43JV4YufW23gm=Uwr7Lkj+p0yKctKHxNm1rwFC+_DQ@mail.gmail.com
Backpatch-through: 18

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7b98c5536818287998ca868dc96a8dc20ea78f0b

Modified Files
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src/backend/storage/aio/README.md | 5 +++--
src/include/storage/aio.h         | 2 +-
2 files changed, 4 insertions(+), 3 deletions(-)