scope/lifetime/lifetime_bounds.md to clearly say it uses "where" keyword

[peter-lyons-kehl]

The documentation part of https://doc.rust-lang.org/rust-by-example/scope/lifetime/lifetime_bounds.html (before the code example) doesn't mention where keyword at all. If you follow Rust by Example table of content, and you never saw where and T: ... before, then you may expect the bounds to be a part of the main generic specification, for example fn print<T>(t: T) or fn print_ref<'a, T>(t: &'a T).

Hence, could you make the documentation (the top part) mention where.