nixos/doc: Note that attrsOf is strict in its values

nixos/doc/manual/development/option-types.xml

···
       An attribute set of where all the values are of
       <replaceable>t</replaceable> type. Multiple definitions result in the
       joined attribute set.
+      <note><para>
+       This type is <emphasis>strict</emphasis> in its values, which in turn
+       means attributes cannot depend on other attributes. See <varname>
+       types.lazyAttrsOf</varname> for a lazy version.
+      </para></note>
      </para>
     </listitem>
    </varlistentry>