Give users some guidance on how to handle missing default cases.

In light of this discussion [1], I think it would be a good idea to explicitly tell the user to put `assert(0)` in their default case, if they don't want to handle it.

Feel free to suggest a better phrasing or a nicer formatting.

[1] http://forum.dlang.org/thread/mailman.122.1392565409.6445.digitalmars-d@puremagic.com?page=6#post-ldvv6r:2431ha:241:40digitalmars.com