Docs for `for` keyword should mention `for<'a>`

[SoniEx2]

Docs for for keyword should mention for<'a> (I guess issue titles still can't have code :/)

[frewsxcv]

FYI, there are some existing docs about for<'a> in the Reference:

• https://doc.rust-lang.org/nightly/reference/trait-bounds.html#higher-ranked-trait-bounds

[Centril]

@SoniEx2 a PR would be welcome.