update the book for lifetime elision in `impl` headers

[steveklabnik]

This issue tracks inclusion of information about lifetime elision in impl headers into the Book. This will happen in the 2018 edition.

Tracking issue: rust-lang/rust#15872

[steveklabnik]

This is ready as per rust-lang/rust#49251

[steveklabnik]

This feature is not yet stable, de-milestoning

[steveklabnik]

I had the wrong tracking issue, now updated with the right one.