pgsql: Update "don't truncate with failsafe" rationale.

Update "don't truncate with failsafe" rationale.

There is a very good (though non-obvious) reason to avoid relation
truncation during a VACUUM that has triggered the failsafe mechanism,
which was missed before now. Update related comments, so this isn't
forgotten.

Reported-By: John Naylor <john.naylor@enterprisedb.com>
Discussion: /messages/by-id/CAFBsxsFiMPxQ-dHZ8tOgktn=+ffeJT3+GinZ4zdOGbmAnCYadA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/988ffc30637ce2b7c4ff984513f4695372602ce2

Modified Files
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src/backend/access/heap/vacuumlazy.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)