pgsql: Improve documentation for pg_atomic_unlocked_write_u32()

Improve documentation for pg_atomic_unlocked_write_u32()

After my recent commit 7902a47c20b, Nathan noticed that
pg_atomic_unlocked_write_u64() was not accurately described by the comments
for the 32bit version. Turns out the 32bit version has suffered from
copy-and-paste-itis since its introduction. Fix.

Reported-by: Nathan Bossart <nathandbossart(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Nathan Bossart <nathandbossart(at)gmail(dot)com>
Discussion: https://postgr.es/m/aTGt7q4Jvn97uGAx@nathan

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/aa749bde323364039f369784071f315ad53c1325

Modified Files
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src/include/port/atomics.h | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)