pgsql: Doc: fix high availability solutions comparison.

Doc: fix high availability solutions comparison.

In "High Availability, Load Balancing, and Replication" chapter,
certain descriptions of Pgpool-II were not correct at this point. It
does not need conflict resolution. Also "Multiple-Server Parallel
Query Execution" is not supported anymore.

Discussion: /messages/by-id/20200726.230128.53842489850344110.t-ishii@sraoss.co.jp
Author: Tatsuo Ishii
Reviewed-by: Bruce Momjian
Backpatch-through: 9.5

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9baf5d639261d72a2f86311fde93f612f7ede9dc

Modified Files
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doc/src/sgml/high-availability.sgml | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)