added MESA_GLX_ALPHA_BITS env var
diff --git a/docs/envvars.html b/docs/envvars.html
index 60cfd3f..b0c784e 100644
--- a/docs/envvars.html
+++ b/docs/envvars.html
@@ -37,6 +37,7 @@
   "window" to render into a window, or "disable" to disable the Glide driver.
 <li>MESA_GLX_FORCE_ALPHA - if set, forces RGB windows to have an alpha channel.
 <li>MESA_GLX_DEPTH_BITS - specifies default number of bits for depth buffer.
+<li>MESA_GLX_ALPHA_BITS - specifies default number of bits for alpha channel.
 </ul>