docs: Document shader capturing environment variables.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>

docs/envvars.html

@@ -133,6 +133,8 @@ that variable is set), or else within .cache/mesa within the user's
 home directory.
 <li>MESA_GLSL - <a href="shading.html#envvars">shading language compiler options</a>
 <li>MESA_NO_MINMAX_CACHE - when set, the minmax index cache is globally disabled.
+<li>MESA_SHADER_CAPTURE_PATH - see <a href="shading.html#capture">Capturing Shaders</a></li>
+<li>MESA_SHADER_DUMP_PATH and MESA_SHADER_READ_PATH - see <a href="shading.html#replacement">Experimenting with Shader Replacements</a></li>
 </ul>