doc: Add secondary index entries for vacuum-related parameters.

For parameters that exist as both configuration and storage options,
the documentation typically includes secondary index entries to
help users distinguish and locate the relevant references easily.

However, such index entries were missing for vacuum_truncate and
vacuum_max_eager_freeze_failure_rate, both introduced in v18.

This commit adds appropriate secondary index terms for these parameters
to ensure consistency with other parameters and improve usability of
the documentation index.

Author: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/e95c899a-2aeb-45b7-8fd3-7a27dcdb475b@oss.nttdata.com

doc/src/sgml/config.sgml

@@ -9340,7 +9340,8 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       <varlistentry id="guc-vacuum-truncate" xreflabel="vacuum_truncate">
        <term><varname>vacuum_truncate</varname> (<type>boolean</type>)
        <indexterm>
-        <primary><varname>vacuum_truncate</varname> configuration parameter</primary>
+        <primary><varname>vacuum_truncate</varname></primary>
+        <secondary>configuration parameter</secondary>
        </indexterm>
        </term>
        <listitem>
@@ -9544,7 +9545,8 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
      <varlistentry id="guc-vacuum-max-eager-freeze-failure-rate" xreflabel="vacuum_max_eager_freeze_failure_rate">
       <term><varname>vacuum_max_eager_freeze_failure_rate</varname> (<type>floating point</type>)
       <indexterm>
-       <primary><varname>vacuum_max_eager_freeze_failure_rate</varname> configuration parameter</primary>
+       <primary><varname>vacuum_max_eager_freeze_failure_rate</varname></primary>
+       <secondary>configuration parameter</secondary>
       </indexterm>
       </term>
       <listitem>

doc/src/sgml/ref/create_table.sgml

@@ -1694,7 +1694,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
    <varlistentry id="reloption-vacuum-truncate" xreflabel="vacuum_truncate">
     <term><literal>vacuum_truncate</literal>, <literal>toast.vacuum_truncate</literal> (<type>boolean</type>)
     <indexterm>
-     <primary><varname>vacuum_truncate</varname> storage parameter</primary>
+     <primary><varname>vacuum_truncate</varname></primary>
+     <secondary>storage parameter</secondary>
     </indexterm>
     </term>
     <listitem>