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 <[email protected]>
Reviewed-by: Tom Lane <[email protected]>
Reviewed-by: Nathan Bossart <[email protected]>
Discussion: https://postgr.es/m/aTGt7q4Jvn97uGAx@nathan