Update for Termination trait

[ehuss]

The Termination trait is now stable. There are several places in the reference that should be updated:

• The return type of main discussed here: https://github.com/rust-lang/reference/blob/master/src/crates-and-source-files.md#main-functions
• The return type of #[test] functions discussed here: https://github.com/rust-lang/reference/blob/master/src/attributes/testing.md#the-test-attribute
• I would add it to the Special types and traits chapter. Just mention it as the return type of the above two points, and link to them.