Update rustdoc book to suggest using Termination trait instead of hidden ‘foo’ function

[frewsxcv]

Towards the end of this section in the rustdoc book, we suggest wrapping functions that utilize ? with fn foo() -> io::Result<()> { .. }.

Now that the Termination trait has stabilized, we can recommend using that instead of this hidden foo function. For examples of what I mean, see this pull request.

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@steveklabnik is happy to mentor anyone who wants to do this. Here's the basic outline of what needs to happen:

Here's the file that needs to be modified: https://github.com/rust-lang/rust/blob/master/src/doc/rustdoc/src/documentation-tests.md

The part that starts with:

Another case where the use of # is handy is when you want to ignore error handling. Lets say you want the following,

This should be moved into its own section, "Using ? in doc tests", and instead of wrapping it in a function, it should suggest something similar to the diff above.

If you have any questions about this, don't hesitate to ask in a comment below!