doc: add 'vec4' option in INTEL_DEBUG

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

docs/envvars.html

@@ -163,6 +163,7 @@ See the <a href="xlibdriver.html">Xlib software driver page</a> for details.
    <li>blorp - emit messages about the blorp operations (blits &amp; clears)</li>
    <li>nodualobj - suppress generation of dual-object geometry shader code</li>
    <li>optimizer - dump shader assembly to files at each optimization pass and iteration that make progress</li>
+   <li>vec4 - force vec4 mode in vertex shader</li>
 </ul>
 </ul>