pgsql: Improve comment for tricky aspect of index-only scans.

Improve comment for tricky aspect of index-only scans.

Index-only scans avoid taking a lock on the VM buffer, which would
cause a lot of contention. To be correct, that requires some intricate
assumptions that weren't completely documented in the previous
comment.

Reviewed by Robert Haas.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/35c0cd3b05b0be18dc2d049c33b38a2d13993ffe

Modified Files
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src/backend/executor/nodeIndexonlyscan.c |   34 ++++++++++++++++++++++--------
1 file changed, 25 insertions(+), 9 deletions(-)