Improve some comments in parser table construction.

* src/LR0.c (new_itemsets): Explain sorting of itemset and kernel_base.
(generate_states): Don't mention ruleset, which is internal to closure.
* src/closure.c (closure): Explain sorting of core and itemset, which
is required for this function to behave correctly.
* src/closure.h (closure): Mention sorting.

src/closure.c

@@ -213,6 +213,8 @@ closure (item_number *core, size_t n)
     if (ISVAR (ritem[core[c]]))
       bitset_or (ruleset, ruleset, FDERIVES (ritem[core[c]]));
 
+  /* core is sorted on item number from ritem, which is sorted on rule number.
+     Compute itemset with the same sort.  */
   nitemset = 0;
   c = 0;
   BITSET_FOR_EACH (iter, ruleset, ruleno, 0)