pgsql: Fix outdated comment that talked about seek position of WAL file

Fix outdated comment that talked about seek position of WAL file.

Since commit c24dcd0cfd, we have been using pg_pread() to read the WAL
file, which doesn't change the seek position (unless we fall back to
the implementation in src/port/pread.c). Update comment accordingly.

Backpatch-through: 12, where we started to use pg_pread()

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f98635ad6295ad142779756e60325a8e7f825909

Modified Files
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src/backend/access/transam/xlog.c | 8 +++-----
1 file changed, 3 insertions(+), 5 deletions(-)