Fully document all forms of lifetime elision

[Gankra]

The ownership guide goes into it briefly, but does not for example give the most important one:

foo(&a, &b, &c, ..) -> &z is an elision of foo<'a, 'b, 'c, ..>(&'a a, &'b b, &'c c, ..) &'a z (output gets lifetime of first input).

Not sure if there are other elisions left in the wild.

[steveklabnik]

https://github.com/rust-lang/rfcs/blob/master/text/0141-lifetime-elision.md#the-rules

[mahkoh]

test6.rs:5:31: 5:35 help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `x` or `y`
test6.rs:5 pub fn f(x: &int, y: &int) -> &int {
                                         ^~~~