pgsql: doc: Add note about re-archiving of same WAL files in docs.

doc: Add note about re-archiving of same WAL files in docs.

The server may attempt to re-archive a WAL file that was previously archived.
This commit adds the note about how an archive library should handle such
a re-archiving.

Author: Nathan Bossart
Reviewed-by: David Steele, Kyotaro Horiguchi
Discussion: https://postgr.es/m/CA+TgmoaeTe4oUT39A4nt=8LD6UJ5u0vcmGc5+Aksn-4oKRb8-w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3cabe45a819f8a2a282d9d57e45f259c84e97c3f

Modified Files
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doc/src/sgml/backup.sgml | 25 ++++++++++++++++++++-----
1 file changed, 20 insertions(+), 5 deletions(-)