Update declarations.xml (#4876)

The bottom type can't be used in a composite type for the same (rather, dual) reason the top type can't.

Author: MorganLOCode

language/types/declarations.xml

@@ -359,7 +359,7 @@ NULL
      </listitem>
      <listitem>
       <simpara>
-       Using <type>mixed</type> results in an error.
+       Using <type>mixed</type> or <type>never</type> results in an error.
       </simpara>
      </listitem>
      <listitem>