needless_doctest_main triggers inside of multiline strings

[udoprog]

The lint triggers for fn main()'s inside of multiline strings, like these:

/// ```
/// let string = r#"
/// fn main() {
///     // things
/// }
/// "#;
/// ```
fn my_documented_function() {
}

With:

warning: needless `fn main` in doctest
  --> crates\rune\src\ast\decl_file.rs:40:4
   |
40 | /// let string = r#"
   |    ^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(clippy::needless_doctest_main)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_doctest_main

[ebroto]

I'm working on this, I think I'll have a PR soon.