doc: move hash performance item into index section

The requirement to rebuild pg_upgrade-ed hash indexes was kept in the
incompatibilities section.

Reported-by: Amit Kapila

doc/src/sgml/release-10.sgml

@@ -49,26 +49,14 @@
 
 <listitem>
 <!--
-Author: Robert Haas <rhaas@postgresql.org>
-2016-11-30 [6d46f4783] Improve hash index bucket split behavior.
-Author: Robert Haas <rhaas@postgresql.org>
-2017-02-07 [293e24e50] Cache hash index's metapage in rel->rd_amcache.
-Author: Robert Haas <rhaas@postgresql.org>
-2017-02-27 [b0f18cb77] hash: Refactor bucket squeeze code.
-Author: Robert Haas <rhaas@postgresql.org>
-2017-02-27 [30df93f69] hash: Refactor overflow page allocation.
-Author: Robert Haas <rhaas@postgresql.org>
-2017-04-03 [ea69a0dea] Expand hash indexes more gradually.
+Created from "Improve hash bucket split performance" item
 -->
 <para>
-Improve hash bucket split performance by reducing locking requirements
-(Amit Kapila, Mithun Cy)
+<application>pg_upgrade</>-ed hash indexes from previous major Postgres versions must be rebuilt.
 </para>
 
 <para>
-Also cache hash index meta-information for faster lookups.  Additional
-hash performance improvements have also been made.  <application>pg_upgrade</>'d hash
-indexes from previous major Postgres versions must be rebuilt.
+Major hash index performance improvements necessitated this requirement.
 </para>
 </listitem>
 
@@ -475,6 +463,30 @@ warning message about their use.
 </para>
 </listitem>
 
+<listitem>
+<!--
+Author: Robert Haas <rhaas@postgresql.org>
+2016-11-30 [6d46f4783] Improve hash index bucket split behavior.
+Author: Robert Haas <rhaas@postgresql.org>
+2017-02-07 [293e24e50] Cache hash index's metapage in rel->rd_amcache.
+Author: Robert Haas <rhaas@postgresql.org>
+2017-02-27 [b0f18cb77] hash: Refactor bucket squeeze code.
+Author: Robert Haas <rhaas@postgresql.org>
+2017-02-27 [30df93f69] hash: Refactor overflow page allocation.
+Author: Robert Haas <rhaas@postgresql.org>
+2017-04-03 [ea69a0dea] Expand hash indexes more gradually.
+-->
+<para>
+Improve hash bucket split performance by reducing locking requirements
+(Amit Kapila, Mithun Cy)
+</para>
+
+<para>
+Also cache hash index meta-information for faster lookups.  Additional
+hash performance improvements have also been made.
+</para>
+</listitem>
+
 <listitem>
 <!--
 Author: Robert Haas <rhaas@postgresql.org>