freedreno/ir3: Add comments about shader key/gen

I had forgotton on which gens these where used on (which is important if
you need to know which shader stages use these).. expand the comments a
bit.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9394>

src/freedreno/ir3/ir3_shader.h

@@ -325,10 +325,10 @@ struct ir3_shader_key {
 		uint32_t global;
 	};
 
-	/* bitmask of ms shifts */
+	/* bitmask of ms shifts (a3xx) */
 	uint32_t vsamples, fsamples;
 
-	/* bitmask of samplers which need astc srgb workaround: */
+	/* bitmask of samplers which need astc srgb workaround (a4xx+a5xx): */
 	uint16_t vastc_srgb, fastc_srgb;
 };