pgsql: Fix documentation bug related to backup history file.

Fix documentation bug related to backup history file.

The backup history file has been no longer necessary for recovery
since the version 9.0. It's now basically just for informational purpose.
But previously the documentations still described that a recovery
requests the backup history file to proceed. The commit fixes this
documentation bug.

Back-patch to all supported versions.

Author: Yugo Nagata
Reviewed-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/20180626174752.0ce505e3.nagata@sraoss.co.jp

Branch
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REL9_3_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/833397e73e2e8d8e37aa38b31e00320161bb6ff8

Modified Files
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doc/src/sgml/backup.sgml            | 2 +-
doc/src/sgml/high-availability.sgml | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)