added MESA_GLX_FORCE_ALPHA env var
diff --git a/docs/envvars.html b/docs/envvars.html
index 39bd725..7cda0e4 100644
--- a/docs/envvars.html
+++ b/docs/envvars.html
@@ -31,6 +31,7 @@
 <li>MESA_GLX_FORCE_CI - if set, force GLX to treak 8bpp visuals as CI visuals
 <li>MESA_GLX_FX - set to either "fullscreen" for full-screen rendering,
   "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.
 </ul>