docs: document MESA_GLSL=errors keyword

Added with commit 0161691f35, still checked on shaderapi.c
_mesa_get_shader_flag method.

Fixes: 0161691f35 "mesa: add GLSL_REPORT_ERRORS debug flag"
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

docs/shading.html

@@ -59,6 +59,7 @@ execution.  These are generally used for debugging.
 <li><b>nopfrag</b> - force fragment shader to be a simple shader that passes
     through the color attribute.
 <li><b>useprog</b> - log glUseProgram calls to stderr
+<li><b>errors</b> - GLSL compilation and link errors will be reported to stderr.
 </ul>
 <p>
 Example:  export MESA_GLSL=dump,nopt