Add a note about Higher-Ranked Trait Bounds in docs on Closures.

[vvanders]

I hit a snag with lifetimes a few days ago and it wasn't until @birkenfeld pointed out Higher-Ranked Trait Bounds that I was able to solve the issue involving lifetimes on closure traits. This adds a small section in the book so that other users can find it.

r? @steveklabnik

[rust-highfive]

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[steveklabnik]

Rust, not rust

[steveklabnik]

Thanks so much for this @vvanders ! I have one small grammar nit, and also, you need to add some annotations:

failures:

---- Taking_closures_as_arguments_5 stdout ----
    <anon>:4:2: 4:2 error: this file contains an un-closed delimiter
<anon>:4 }
          ^
<anon>:1:11: 1:12 help: did you mean to close this delimiter?
<anon>:1 fn main() {
                   ^
<anon>:3:21: 3:23 error: expected type, found `32`
<anon>:3     where F: Fn(&'a 32) -> i32 {
                             ^~
<anon>:3:23: 3:24 error: expected one of `+`, `,`, `->`, `::`, or `{`, found `)`
<anon>:3     where F: Fn(&'a 32) -> i32 {
                               ^
error: aborting due to previous error(s)
thread 'Taking_closures_as_arguments_5' panicked at 'Box<Any>', src/librustc/session/mod.rs:167
note: Run with `RUST_BACKTRACE=1` for a backtrace.

---- Taking_closures_as_arguments_6 stdout ----
    <anon>:4:2: 4:2 error: this file contains an un-closed delimiter
<anon>:4 }
          ^
<anon>:1:11: 1:12 help: did you mean to close this delimiter?
<anon>:1 fn main() {
                   ^
<anon>:3:29: 3:31 error: expected type, found `32`
<anon>:3     where F: for<'a> Fn(&'a 32) -> i32 {
                                     ^~
<anon>:3:31: 3:32 error: expected one of `+`, `,`, `->`, `::`, or `{`, found `)`
<anon>:3     where F: for<'a> Fn(&'a 32) -> i32 {
                                       ^
error: aborting due to previous error(s)
thread 'Taking_closures_as_arguments_6' panicked at 'Box<Any>', src/librustc/session/mod.rs:167


failures:
    Taking_closures_as_arguments_5
    Taking_closures_as_arguments_6

The examples get run as tests, so they don't work. Adding ignore will fix it:

```ignore

Update it with that, and this should be good to go 👍

[vvanders]

Thanks for the feedback and happy to help where I can. I'll see if I can address those and get an updated PR sent.

[vvanders]

Hey @steveklabnik, just wanted to check in here and make sure everything is good. Let me know if there's any further feedback or if you think this is okay to merge.

[steveklabnik]

@vvanders ah! Sorry about that. GitHub doesn't send a message when you push new commits; I didn't know you'd done so.

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit 64feba0 has been approved by steveklabnik

[vvanders]

No worries, glad to have it merged :).