doc: Move documentation of md5_password_warnings to a better place

Commit db6a4a985b categorized md5_password_warnings as an
authentication setting, and the placement in postgresql.conf.sample
matches that, but in the documentation it ended up under logging
settings, which isn't unreasonable but inconsistent.  This moves the
documentation chunk to authentication settings as well.

doc/src/sgml/config.sgml

@@ -1155,6 +1155,22 @@ include_dir 'conf.d'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-md5-password-warnings" xreflabel="md5_password_warnings">
+      <term><varname>md5_password_warnings</varname> (<type>boolean</type>)
+      <indexterm>
+       <primary><varname>md5_password_warnings</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Controls whether a <literal>WARNING</literal> about MD5 password
+        deprecation is produced when a <command>CREATE ROLE</command> or
+        <command>ALTER ROLE</command> statement sets an MD5-encrypted password.
+        The default value is <literal>on</literal>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-krb-server-keyfile" xreflabel="krb_server_keyfile">
       <term><varname>krb_server_keyfile</varname> (<type>string</type>)
       <indexterm>
@@ -8118,22 +8134,6 @@ log_line_prefix = '%m [%p] %q%u@%d/%a '
       </listitem>
      </varlistentry>
 
-     <varlistentry id="guc-md5-password-warnings" xreflabel="md5_password_warnings">
-      <term><varname>md5_password_warnings</varname> (<type>boolean</type>)
-      <indexterm>
-       <primary><varname>md5_password_warnings</varname> configuration parameter</primary>
-      </indexterm>
-      </term>
-      <listitem>
-       <para>
-        Controls whether a <literal>WARNING</literal> about MD5 password
-        deprecation is produced when a <command>CREATE ROLE</command> or
-        <command>ALTER ROLE</command> statement sets an MD5-encrypted password.
-        The default value is <literal>on</literal>.
-       </para>
-      </listitem>
-     </varlistentry>
-
      </variablelist>
     </sect2>
      <sect2 id="runtime-config-logging-csvlog">