Describe each contrib module in its SGML section title

The original titles only had the module name, which is not very useful
when scanning the list.  By adding a very brief description to each
title, the table of contents becomes friendlier.

Also amend the introduction in the "additional modules" appendix, using
the word "Extension" more extensively.  Nowadays, almost all contrib
modules are extensions, so this is also helpful.

Author: Karl O. Pinc <kop@karlpinc.com>
Reviewed-by: Brar Piening <brar@gmx.de>
Discussion: https://postgr.es/m/20230102180015.372995a9@slate.karlpinc.com

Author: alvherre

doc/src/sgml/adminpack.sgml

@@ -1,7 +1,7 @@
 <!-- doc/src/sgml/adminpack.sgml -->
 
 <sect1 id="adminpack" xreflabel="adminpack">
- <title>adminpack</title>
+ <title>adminpack &mdash; pgAdmin support toolpack</title>
 
  <indexterm zone="adminpack">
   <primary>adminpack</primary>

doc/src/sgml/amcheck.sgml

@@ -1,7 +1,7 @@
 <!-- doc/src/sgml/amcheck.sgml -->
 
 <sect1 id="amcheck" xreflabel="amcheck">
- <title>amcheck</title>
+ <title>amcheck &mdash; tools to verify table and index consistency</title>
 
  <indexterm zone="amcheck">
   <primary>amcheck</primary>

doc/src/sgml/auth-delay.sgml

@@ -1,7 +1,7 @@
 <!-- doc/src/sgml/auth-delay.sgml -->
 
 <sect1 id="auth-delay" xreflabel="auth_delay">
- <title>auth_delay</title>
+ <title>auth_delay &mdash; pause on authentication failure</title>
 
  <indexterm zone="auth-delay">
   <primary>auth_delay</primary>

doc/src/sgml/auto-explain.sgml

@@ -1,7 +1,7 @@
 <!-- doc/src/sgml/auto-explain.sgml -->
 
 <sect1 id="auto-explain" xreflabel="auto_explain">
- <title>auto_explain</title>
+ <title>auto_explain &mdash; log execution plans of slow queries</title>
 
  <indexterm zone="auto-explain">
   <primary>auto_explain</primary>

doc/src/sgml/basebackup-to-shell.sgml

@@ -1,7 +1,7 @@
 <!-- doc/src/sgml/basebackup-to-shell.sgml -->
 
 <sect1 id="basebackup-to-shell" xreflabel="basebackup_to_shell">
- <title>basebackup_to_shell</title>
+ <title>basebackup_to_shell &mdash; example "shell" pg_basebackup module</title>
 
  <indexterm zone="basebackup-to-shell">
   <primary>basebackup_to_shell</primary>

doc/src/sgml/basic-archive.sgml

@@ -1,7 +1,7 @@
 <!-- doc/src/sgml/basic-archive.sgml -->
 
 <sect1 id="basic-archive" xreflabel="basic_archive">
- <title>basic_archive</title>
+ <title>basic_archive &mdash; an example WAL archive module</title>
 
  <indexterm zone="basic-archive">
   <primary>basic_archive</primary>

doc/src/sgml/bloom.sgml

@@ -1,7 +1,7 @@
 <!-- doc/src/sgml/bloom.sgml -->
 
 <sect1 id="bloom" xreflabel="bloom">
- <title>bloom</title>
+ <title>bloom &mdash; bloom filter index access method</title>
 
  <indexterm zone="bloom">
   <primary>bloom</primary>

doc/src/sgml/btree-gin.sgml

@@ -1,14 +1,14 @@
 <!-- doc/src/sgml/btree-gin.sgml -->
 
 <sect1 id="btree-gin" xreflabel="btree_gin">
- <title>btree_gin</title>
+ <title>btree_gin &mdash; GIN operator classes with B-tree behavior</title>
 
  <indexterm zone="btree-gin">
   <primary>btree_gin</primary>
  </indexterm>
 
  <para>
-  <filename>btree_gin</filename> provides sample GIN operator classes that
+  <filename>btree_gin</filename> provides GIN operator classes that
   implement B-tree equivalent behavior for the data types
   <type>int2</type>, <type>int4</type>, <type>int8</type>, <type>float4</type>,
   <type>float8</type>, <type>timestamp with time zone</type>,

doc/src/sgml/btree-gist.sgml

@@ -1,7 +1,7 @@
 <!-- doc/src/sgml/btree-gist.sgml -->
 
 <sect1 id="btree-gist" xreflabel="btree_gist">
- <title>btree_gist</title>
+ <title>btree_gist &mdash; GiST operator classes with B-tree behavior</title>
 
  <indexterm zone="btree-gist">
   <primary>btree_gist</primary>

doc/src/sgml/citext.sgml

@@ -1,7 +1,7 @@
 <!-- doc/src/sgml/citext.sgml -->
 
 <sect1 id="citext" xreflabel="citext">
- <title>citext</title>
+ <title>citext &mdash; a case-insensitive character string type</title>
 
  <indexterm zone="citext">
   <primary>citext</primary>