Part of rust-lang#29378.

Part of rust-lang#29378 .

- In particular explains *why* we would use the builder instead of
  the free function.
- Changes the parent-child explanation for a spawned-caller.
- Add link to `io::Result` in `thread::Builder`
- Corrects the `thread::Builder::spawn` documentation.

Make diagnostic note for existing method with unsatisfied trait bounds
multiline for cleaner output.

```
  = note: the method `count` exists but the following trait bounds were not satisfied:
          `[closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53] : std::ops::FnMut<(&_,)>`
          `std::iter::Filter<std::iter::Fuse<std::iter::Once<&str>> [closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53]> : std::iter::Iterator`

Before:

```
  = note: the method `count` exists but the following trait bounds were not satisfied: `[closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53] : std::ops::FnMut<(&_,)>`, `std::iter::Filter<std::iter::Fuse<std::iter::Once<&str>>, [closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53]> : std::iter::Iterator`
```

Part of rust-lang#29378 .

…r=arielb1

Make unsatisfied trait bounds note multiline

Make diagnostic note for existing method with unsatisfied trait bounds
multiline for cleaner output.

```
  = note: the method `count` exists but the following trait bounds were not satisfied:
          `[closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53] : std::ops::FnMut<(&_,)>`
          `std::iter::Filter<std::iter::Fuse<std::iter::Once<&str>> [closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53]> : std::iter::Iterator`
```

Before:

```
  = note: the method `count` exists but the following trait bounds were not satisfied: `[closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53] : std::ops::FnMut<(&_,)>`, `std::iter::Filter<std::iter::Fuse<std::iter::Once<&str>>, [closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53]> : std::iter::Iterator`
```

…umeGomez

[Doc] Explain why `thread::yield_now` could be used.

Part of rust-lang#29378.

r? @steveklabnik

…eGomez

[Doc] Implrove `thread::Builder`'s doc.

Part of rust-lang#29378 .

- Explains *why* we would use the builder instead ofthe free function.
- Changes the parent-child explanation for a spawned-caller in `thread::Builder::spawn`
- Adds a link to `io::Result` in `thread::Builder`
- Corrects the return type doc in `thread::Builder::spawn`

r? @rust-lang/docs

[Doc] Add links to the `thread::LocalKey` doc.

Part of rust-lang#29378 .

I do not know exactly what should be done for the `cleanup` part, if you have any idea I'll gladly do it.

r? @rust-lang/docs

…umeGomez

rustdoc: Display `extern "C" fn` instead of `extern fn`

It was decided in rust-lang/style-team#52 to be explicit about the ABI so rustdoc should follow suit.

Fix regression in `macro_rules!` name matching

Fixes rust-lang#41803.
r? @nrc

improve collapse toggle render (css)

The `[-]` toggle for functions in docs _seems_ too big. It's just an impression, but it's something I noticed long time ago (maybe I have bad taste). I never thought to fix it, but, today I think: "Ok, why not suggest it.". Feel free to close without explanation!

Preview changes below:

From this:

<img width="1003" alt="capture d ecran 2017-05-15 a 17 14 45" src="https://cloud.githubusercontent.com/assets/1575946/26064816/5c84de86-3992-11e7-976b-41c625cace0f.png">

To this:

<img width="996" alt="capture d ecran 2017-05-15 a 17 15 02" src="https://cloud.githubusercontent.com/assets/1575946/26064854/78325dac-3992-11e7-88f6-2c43db43421c.png">

Typo