Added some more tests, comments, notes.

Commmit [d907079d5b].

Regarding documentation question: I see no problem adding disclaimers about transform limitations and will accept PRs.

Wrt start set for `drop unrealizable` ... I can see that this can ignore unreachable. Even if they are realizable they would either go away later in the minimize, or simply be ignored during execution of either grammar interpreter or a generated parser.