Fix outdated comment that talked about seek position of WAL file.

Fix outdated comment that talked about seek position of WAL file.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