make Future scaladoc examples up-to-date and compilable

Hi There, 
I've stumbled upon some examples in Future's scaladoc which are obsolete and will not compile. That's definitely not a critical thing, but I think it should go to 2.10.x b/c Future is a new "feature" class in 2.10 and lots of users who are going to migrate to 2.10 can find that confusing. 
There are also two unrelated minor corrections to ArrayBuffer and BigInt scaladocs.
~
Eugene

review by @heathermiller, @axel22