update docs with removal of libgreen

[steveklabnik]

I should make a pass at the tasks guide as well as http://doc.rust-lang.org/guide.html#tasks to make sure I don't imply things that are wrong.

. Tasks are lightweight,

seems bad.

[nathanaschbacher]

When's the "thread" renaming coming? Seems like a perfect time to tackle the whole lot.

[sinistersnare]

@aturon, what is the status on that?

[aturon]

The renaming to thread (and total revamp of the API) is coming very soon -- I have a branch, but it's blocked on #19274 at least. Should land in the next week or two.