st: Fix comments in commit be6d584de4

This patch is adding comments as suggested by Ilia in MR

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13423>

src/mesa/state_tracker/st_glsl_to_tgsi.cpp

@@ -6686,7 +6686,9 @@ st_translate_program(
 
    if (proginfo->DualSlotInputs != 0) {
       /* adjust attrToIndex to include placeholder for second
-       * part of a double attribute
+       * part of a double attribute.
+       * Following code is basically matching behavior of
+       * util_lower_uint64_vertex_elements
        */
       numInputs = 0;
       for (unsigned attr = 0; attr < VERT_ATTRIB_MAX; attr++) {