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_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ef2deca1b77ebcbd554eaa8dfcd5e653ea67593b

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(-)