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: https://postgr.es/m/20200726.230128.53842489850344110.t-ishii%40sraoss.co.jp
Author: Tatsuo Ishii
Reviewed-by: Bruce Momjian
Backpatch-through: 9.5

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8680303f9b7b7b5c5a4b3116fc2d2d7e7f2eac7a

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