pgsql: Reword overly-optimistic comment about backup checksum verificat

Reword overly-optimistic comment about backup checksum verification.

The comment implies that a single retry is sufficient to avoid
spurious checksum failures, but in fact no number of retries is
sufficient for that purpose. Update the comment accordingly.

Patch by me, reviewed by Michael Paquier.

Discussion: http://postgr.es/m/CA+TgmoZ_fFAoU6mrHt9QBs+dcYhN6yXenGTTMRebZNhtwPwHyg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e76cbb6cd64d9c9cdf76a56318ba5249bf694b69

Modified Files
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src/backend/backup/basebackup.c | 20 +++++++++++++++-----
1 file changed, 15 insertions(+), 5 deletions(-)