updated external core frontend info
<listitem>
<para>The compiler can now read in files containing Core
syntax (such as those produced by the
- <option>-fext-core</option> option) and compile them. This is
- enabled with the <option>-fcore</option> flag.</para>
+ <option>-fext-core</option> option) and compile them. Input
+ files with the <literal>.hcr</literal> file extension are
+ assumed to contain Core syntax.</para>
</listitem>
</itemizedlist>
</sect2>