Document how to best write examples that use try for error handling

[killercup]

@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!.

[steveklabnik]

I'm willing to mentor anyone who wants to do this.

I'll take this on.

[steveklabnik]

@EFindlay great! Let me know if you need any help.

I reduced the example to one try! Maybe its better to have two try! s. I'm not sure.