docs: document MESA_EXTENSION_OVERRIDE env var
diff --git a/docs/envvars.html b/docs/envvars.html
index 7fd9fe7..6d7a321 100644
--- a/docs/envvars.html
+++ b/docs/envvars.html
@@ -25,6 +25,9 @@
<li>MESA_TNL_PROG - if set, implement conventional vertex transformation
operations with vertex programs (intended for developers only).
Setting this variable automatically sets the MESA_TEX_PROG variable as well.
+<li>MESA_EXTENSION_OVERRIDE - can be used to enable/disable extensions.
+A value such as "GL_EXT_foo -GL_EXT_bar" will enable the GL_EXT_foo extension
+and disable the GL_EXT_bar extension.
</ul>
<p>