Document how to best write examples that use try for error handling #29234
Description
@steveklabnik wrote on reddit, this this is a common style in std:
/// A doc test using `try!`:
///
/// ```
/// # fn f() -> io::Result<()> {
/// let x = try!(package.open()); // danger, might explode
/// x.insert("valuables", 42)
/// try!(x.close_and_reactivate());
/// # Ok(())
/// # }
/// # f();
/// ```
We should add that to the documentation chapter of TRPL and maybe also to the docs of try!.