mesa: adds some comments regarding MESA_GLES_VERSION_OVERRIDE usage

Fixes: 03fd6704 ("mesa: Add support for a new override string
MESA_GLES_VERSION_OVERRIDE")

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 7cf39320)