Add subsection headers in pageinspect documentation

extracted from a patch from Jesper Pedersen <jesper.pedersen@redhat.com>

doc/src/sgml/pageinspect.sgml

@@ -14,7 +14,7 @@
  </para>
 
  <sect2>
-  <title>Functions</title>
+  <title>General Functions</title>
 
   <variablelist>
    <varlistentry>
@@ -160,7 +160,36 @@ test=# SELECT * FROM heap_page_item_attrs(get_raw_page('pg_class', 0), 'pg_class
      </para>
     </listitem>
    </varlistentry>
-   
+
+   <varlistentry>
+    <term>
+     <function>fsm_page_contents(page bytea) returns text</function>
+     <indexterm>
+      <primary>fsm_page_contents</primary>
+     </indexterm>
+    </term>
+
+    <listitem>
+     <para>
+      <function>fsm_page_contents</function> shows the internal node structure
+      of a FSM page. The output is a multiline string, with one line per
+      node in the binary tree within the page. Only those nodes that are not
+      zero are printed. The so-called "next" pointer, which points to the
+      next slot to be returned from the page, is also printed.
+     </para>
+     <para>
+      See <filename>src/backend/storage/freespace/README</> for more
+      information on the structure of an FSM page.
+     </para>
+    </listitem>
+   </varlistentry>
+  </variablelist>
+ </sect2>
+
+ <sect2>
+  <title>B-tree Functions</title>
+
+  <variablelist>
    <varlistentry>
     <term>
      <function>bt_metap(relname text) returns record</function>
@@ -261,7 +290,13 @@ test=# SELECT * FROM bt_page_items('pg_cast_oid_index', 1);
      </para>
     </listitem>
    </varlistentry>
+  </variablelist>
+ </sect2>
 
+ <sect2>
+  <title>BRIN Functions</title>
+
+  <variablelist>
    <varlistentry>
     <term>
      <function>brin_page_type(page bytea) returns text</function>
@@ -365,7 +400,13 @@ test=# SELECT * FROM brin_page_items(get_raw_page('brinidx', 5),
      </para>
     </listitem>
    </varlistentry>
+  </variablelist>
+ </sect2>
+
+ <sect2>
+  <title>GIN Functions</title>
 
+  <variablelist>
    <varlistentry>
     <term>
      <function>gin_metapage_info(page bytea) returns record</function>
@@ -449,29 +490,6 @@ test=# SELECT first_tid, nbytes, tids[0:5] AS some_tids
      </para>
     </listitem>
    </varlistentry>
-
-   <varlistentry>
-    <term>
-     <function>fsm_page_contents(page bytea) returns text</function>
-     <indexterm>
-      <primary>fsm_page_contents</primary>
-     </indexterm>
-    </term>
-
-    <listitem>
-     <para>
-      <function>fsm_page_contents</function> shows the internal node structure
-      of a FSM page. The output is a multiline string, with one line per
-      node in the binary tree within the page. Only those nodes that are not
-      zero are printed. The so-called "next" pointer, which points to the
-      next slot to be returned from the page, is also printed.
-     </para>
-     <para>
-      See <filename>src/backend/storage/freespace/README</> for more
-      information on the structure of an FSM page.
-     </para>
-    </listitem>
-   </varlistentry>
   </variablelist>
  </sect2>