pgsql: Add a comment documenting the question of whether PrefetchBuffer

Log Message:
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Add a comment documenting the question of whether PrefetchBuffer should
try to protect an already-existing buffer from being evicted.  This was
left as an open issue when the posix_fadvise patch was committed.  I'm
not sure there's any evidence to justify more work in this area, but we
should have some record about it in the source code.

Modified Files:
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    pgsql/src/backend/storage/buffer:
        bufmgr.c (r1.250 -> r1.251)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c?r1=1.250&r2=1.251)