mesa/formatquery: expand NUM_SAMPLE_COUNTS OpenGL ES comment

mesa/formatquery: expand NUM_SAMPLE_COUNTS OpenGL ES comment

For ES 3.0 NUM_SAMPLE_COUNTS spec points that some formats will be
always zero. But on ES 3.1 can be different to zero.

The current code is correctly checking exactly against version 3.0,
but the comment only mentions 3.0 spec. It is clearer mentioning both.

v2: better wording on the comment (Ian Romanick)

Acked-by: Eduardo Lima <elima@igalia.com>
Acked-by: Antia Puentes <apuentes@igalia.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>