Ehance ddoc for std.array.split.

This started with fixing a simple typo, but I decided to go ahead and go all out. If I'm on the right track with this, I'll do more.

I've added an extra paragraph, appropriate DDOC tags (Examples, Params, Returns), and changed the template type parameters to match the type parameters for std.algorithm.splitter. I also renamed the delimiter parameter to 'sep' to match std.array.join which follows this (std.algorthim.splitter calls it 's' instead).

I'm not sure about the description of the returned array. Should it be called an 'array of ranges'?

Finally, I think the version of the template that has the 'alias isTerminator' needs to be documented separately, but I haven't figured out yet what that template alias is doing.