Modify doctest's auto-fn main() to allow Results

This lets the default `fn main()` unwrap any `Result`s, which
allows the use of `?` in most tests without adding it manually.

src/test/rustdoc/process-termination.rs

@@ -0,0 +1,24 @@
+// compile-flags:--test
+
+/// A check of using various process termination strategies
+///
+/// # Examples
+///
+/// ```rust
+/// assert!(true); // this returns `()`, all is well
+/// ```
+///
+/// You can also simply return `Ok(())`, but you'll need to disambiguate the
+/// type using turbofish, because we cannot infer the type:
+///
+/// ```rust
+/// Ok::<(), &'static str>(())
+/// ```
+///
+/// You can err with anything that implements `Debug`:
+///
+/// ```rust,should_panic
+/// Err("This is returned from `main`, leading to panic")?;
+/// Ok::<(), &'static str>(())
+/// ```
+pub fn check_process_termination() {}