docs: document MESA_GLSL=errors keyword

Added with commit 0161691f3518, still checked on shaderapi.c
_mesa_get_shader_flag method.

Fixes: 0161691f3518db310411 "mesa: add GLSL_REPORT_ERRORS debug flag"
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
1 file changed