libsyntax: Explicit error message for sugared doc comments.

Display an explicit message about items missing after sugared doc
comment attributes. References #2789.

 * I tried looking through `parser.rs` for an appropriate location for `expected_item_err` and ended up putting it just above the first use. Is there a better location?
 * Did I add enough test cases? Too many? Should I add more cases for the original error message?
I think this should just check the last one, or possibly list all possibilities.

e.g, talking about doc comments for

```rust
/// Foo
#[inline]
#[foo]
#[bar]
```

is a little weird.