Correct typos and remove unused references from RELEASES.md

I found some typos in the upcoming 1.1 release note. I corrected them,
but I wanted to go further. So I wrote a script that checks the
integrity of the Markdown references, and ran it against `RELEASES.md`.

This commit fixes some trivial cases, but also removes the following
"unused" references:

- [`Iterator::cloned`](http://doc.rust-lang.org/nightly/core/iter/trait.Iterator.html#method.cloned)
- [`thread::scoped`](http://static.rust-lang.org/doc/master/std/thread/fn.scoped.html)
- [`Debug` improvements](https://github.com/rust-lang/rfcs/blob/master/text/0640-debug-improvements.md)
- [Rebalancing coherence.](https://github.com/rust-lang/rfcs/pull/1023)

However, I think there's a possibility that these features might need to
get descriptions as well. How do you feel about it?

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