Rustdoc should emit a warning if a main function is surrounded by non-items in a doctest

[GuillaumeGomez]

For example:

//! ```
//! let x = 12;
//!
//! fn main() {
//!     // this code is never run
//! }
//! ```

In this case, the main function will be wrapped into another main function and won't be run. We should warn about it.