mesa: add new MESA_GLSL flag for printing shader cache debug info

Reviewed-by: Eric Anholt <eric@anholt.net>
diff --git a/docs/shading.html b/docs/shading.html
index b0ed249..e44035a 100644
--- a/docs/shading.html
+++ b/docs/shading.html
@@ -49,6 +49,7 @@
 <li><b>log</b> - log all GLSL shaders to files.
     The filenames will be "shader_X.vert" or "shader_X.frag" where X
     the shader ID.
+<li><b>cache_info</b> - print debug information about shader cache
 <li><b>nopt</b> - disable compiler optimizations
 <li><b>opt</b> - force compiler optimizations
 <li><b>uniform</b> - print message to stdout when glUniform is called