Document the rules for `extern crate` in doc tests

[astraw]

It's not clear from the documentation how to use external crates in doc tests. This should be documented in the testing section of the book.

This is related to rust-lang/cargo#960.

[astraw]

Now I see that this is actually described in the documentation chapter here. However, I was looking at the testing chapter here. So the bug report should now be changed to "cross-referece the doctest part of the documentation chapter from the doctest part of the testing chapter" since I completely missed the relevant section despite looking for it rather extensively.