Add more index entries for reference pages related to prepare queries.

From Alvaro Herrera, editorializing by Neil Conway.

doc/src/sgml/ref/deallocate.sgml

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@@ -18,6 +18,11 @@ PostgreSQL documentation
   <primary>DEALLOCATE</primary>
  </indexterm>
 
+ <indexterm zone="sql-deallocate">
+  <primary>prepared statements</primary>
+  <secondary>removing</secondary>
+ </indexterm>
+
  <refsynopsisdiv>
 <synopsis>
 DEALLOCATE [ PREPARE ] <replaceable class="parameter">plan_name</replaceable>

doc/src/sgml/ref/execute.sgml

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@@ -18,6 +18,11 @@ PostgreSQL documentation
   <primary>EXECUTE</primary>
  </indexterm>
 
+ <indexterm zone="sql-execute">
+  <primary>prepared statements</primary>
+  <secondary>executing</secondary>
+ </indexterm>
+
  <refsynopsisdiv>
 <synopsis>
 EXECUTE <replaceable class="PARAMETER">plan_name</replaceable> [ (<replaceable class="PARAMETER">parameter</replaceable> [, ...] ) ]

doc/src/sgml/ref/explain.sgml

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@@ -18,6 +18,16 @@ PostgreSQL documentation
   <primary>EXPLAIN</primary>
  </indexterm>
 
+ <indexterm zone="sql-explain">
+  <primary>prepared statements</primary>
+  <secondary>showing the query plan</secondary>
+ </indexterm>
+
+ <indexterm zone="sql-explain">
+  <primary>cursor</primary>
+  <secondary>showing the query plan</secondary>
+ </indexterm>
+
  <refsynopsisdiv>
 <synopsis>
 EXPLAIN [ ANALYZE ] [ VERBOSE ] <replaceable class="parameter">statement</replaceable>

doc/src/sgml/ref/prepare.sgml

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@@ -18,6 +18,11 @@ PostgreSQL documentation
   <primary>PREPARE</primary>
  </indexterm>
 
+ <indexterm zone="sql-prepare">
+  <primary>prepared statements</primary>
+  <secondary>creating</secondary>
+ </indexterm>
+
  <refsynopsisdiv>
 <synopsis>
 PREPARE <replaceable class="PARAMETER">plan_name</replaceable> [ (<replaceable class="PARAMETER">datatype</replaceable> [, ...] ) ] AS <replaceable class="PARAMETER">statement</replaceable>