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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master

Details
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https://git.postgresql.org/pg/commitdiff/bbbbc2f8f3f7d628c74317959a2cfe71fea05575

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(-)