pgsql: Fix outdated comment that talked about seek position of WAL file

Fix outdated comment that talked about seek position of WAL file.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_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e89a8e30e0a50e7882ab5a6896a11872bec969e3

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