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

doc/src/sgml/high-availability.sgml

@@ -433,7 +433,7 @@ protocol to make nodes agree on a serializable transactional order.
      <entry align="center">&bull;</entry>
      <entry align="center"></entry>
      <entry align="center">&bull;</entry>
-     <entry align="center"></entry>
+     <entry align="center">&bull;</entry>
      <entry align="center"></entry>
      <entry align="center">&bull;</entry>
     </row>
@@ -475,9 +475,8 @@ protocol to make nodes agree on a serializable transactional order.
      concurrently on a single query.  It is usually accomplished by
      splitting the data among servers and having each server execute its
      part of the query and return results to a central server where they
-     are combined and returned to the user.  <productname>Pgpool-II</>
+     are combined and returned to the user. This can be implemented using the
-     has this capability.  Also, this can be implemented using the
+     <productname>PL/Proxy</productname> tool set.
-     <productname>PL/Proxy</> tool set.
     </para>
 
    </listitem>