pgsql: doc: Fix description about default behavior of recovery_target_t

doc: Fix description about default behavior of recovery_target_timeline.

The default value of recovery_target_timeline was changed in v12,
but the description about the default behavior of that was not updated.

Back-patch to v12 where the default behavior of recovery_target_timeline
was changed.

Author: Benoit Lobréau
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/CAPE8EZ7c3aruEmM24GYkj8y8WmHKD1m9TtPtgCF0nQ3zw4LCkQ@mail.gmail.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c919ed191dc0a6b4d0cbd33f2d42b8c584f35759

Modified Files
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doc/src/sgml/backup.sgml | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)