st/mesa: new comment about updating state vars

src/mesa/state_tracker/st_atom_constbuf.c

@@ -66,6 +66,11 @@ void st_upload_constants( struct st_context *st,
    if (params && params->NumParameters) {
       const uint paramBytes = params->NumParameters * sizeof(GLfloat) * 4;
 
+      /* Update the constants which come from fixed-function state, such as
+       * transformation matrices, fog factors, etc.  The rest of the values in
+       * the parameters list are explicitly set by the user with glUniform,
+       * glProgramParameter(), etc.
+       */
       _mesa_load_state_parameters(st->ctx, params);
 
       /* We always need to get a new buffer, to keep the drivers simple and