Update a TODO comment in mon-cast.cc

It was slightly out of date mentioning a parameter that didn't exist
anymore, and prohibitively complex enough to do that it won't ever
happen without a major rewrite of a huge number of spell functions.

crawl-ref/source/mon-cast.cc

@@ -4680,7 +4680,10 @@ static string _ru_spell_stop_desc(monster &mons)
 /// What spells can the given monster currently use?
 static monster_spells _find_usable_spells(monster &mons)
 {
-    // TODO: make mons param const (requires waste_of_time param to be const)
+    // XX: it would be nice to make the mons param const, but this requires
+    // such significant changes inside _monster_spell_goodness it doesn't seem
+    // viable, as logic there frequently involves calling *real* cast functions
+    // with a tracer param or similar.
 
     monster_spells hspell_pass(mons.spells);