Use `///` for all docs

Hi! Is there any reason why multi-line docs are formatted using `/**`?

For example, the following code:
```rust
#[doc = " This is a single line doc"]
pub type foo = ::std::os::raw::c_int;
#[doc = " This is a multi line doc.\n With way too many lines"]
pub type bar = ::std::os::raw::c_int;
```
is being formatted as
```rust
/// This is a single line doc
pub type foo = ::std::os::raw::c_int;
/** This is a multi line doc.
With way too many lines*/
pub type bar = ::std::os::raw::c_int;
```
and I'd like it to be
```rust
/// This is a single line doc
pub type foo = ::std::os::raw::c_int;
/// This is a multi line doc.
/// With way too many lines
pub type bar = ::std::os::raw::c_int;
```