Improve some comments in parser table construction.

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.