docs: document INTEL_PRECISE_TRIG envvar

v2: use more generic description (Jordan)

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

docs/envvars.html

@@ -187,6 +187,8 @@ See the <a href="xlibdriver.html">Xlib software driver page</a> for details.
    <li>do32 - generate compute shader SIMD32 programs even if workgroup size doesn't exceed the SIMD16 limit</li>
    <li>norbc - disable single sampled render buffer compression</li>
 </ul>
+<li>INTEL_PRECISE_TRIG - if set to 1, true or yes, then the driver prefers
+   accuracy over performance in trig functions.</li>
 </ul>