Fix docs for `fs::create_dir_all()`

So... correct me if I'm wrong, but [`fs::create_dir_all()`](https://docs.rs/async-std/latest/async_std/fs/fn.create_dir_all.html)'s docs seems to have a minor issue - it has the wrong behavior documented:

>An error will be returned in the following situations: path already points to an existing file or directory.

However, async_std's `create_dir_all()` uses std's [`create_dir_all()`](https://doc.rust-lang.org/std/fs/fn.create_dir_all.html) internally, which does NOT return an error from an existing path:

>Recursively create a directory and all of its parent components **_if they are missing._**

and the [`DirBuilder::create()`](https://doc.rust-lang.org/std/fs/struct.DirBuilder.html#method.create) (which `std::fs::create_dir_all()` uses internally) states:

>It is considered an error if the directory already exists **_unless recursive mode is enabled._**

This can be demonstrated with the following code:
```rust
fn main() {
async_std::task::block_on(async {
let x = Path::new("./hello/world");
println!("{:?}", fs::create_dir_all(x).await);
});
}
```

Running the above code twice prints `Ok(())` twice, instead of printing an `Err(...)` after an `Ok(())`.

As a side note, a statement similar to the one in std's `create_dir_all()` documentation exists, but is phrased ambiguously, making it look like only the parent directory creation is optional when paired with the bad documentation:

>Creates a new directory and all of its parents if they are missing.