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_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d06d92cd2d5e3c3dd893ee0d82d14d1f8adbc82b

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(-)