pgsql: Add nbtree README note on index-only scans in recovery.

Add nbtree README note on index-only scans in recovery.

The nbtree README has long claimed that "the interlocking required to
avoid returning incorrect results from non-MVCC scans is not required on
standby nodes". A recent test case proved that the underlying reasoning
was faulty.

There are significant practical challenges around fixing this bug, and
no user has complained about it so far. The old "pin scan" mechanism
prevented the problem but performed very poorly. Since there seems to
be no appetite for resurrecting that mechanism, and since an acceptable
fix hasn't been forthcoming, make a note of the issue for later.

Discussion: https://postgr.es/m/CAH2-Wzkro678xEDOrsxrq-YDVP4w7nXybhVJV2MxrzTj-5OU4w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9e7d152488f197e96f171f0714459c227c3f7693

Modified Files
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src/backend/access/nbtree/README | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)