override index: clarify this doc's purpose

  <summary>

    <p>

      This is an index of the directives that are allowed in .htaccess files for

      each <directive module="core">AllowOverride</directive> setting. To

      determine the set of directives that your server configuration allows

      various <directive module="core">AllowOverride</directive> settings,

      organized by class. Its intended purpose is to help server administrators

      verify the privileges they're granting to .htaccess users. For an overview

      of how .htaccess works, see the

      <a href="../howto/htaccess.html">.htaccess tutorial</a>.

    </p>

    <p>

      To determine the set of directives that your server configuration allows

      .htaccess users to use:

    </p>