Added explanation to trpl of integer types.

[jxcl]

The Rust Programming Language book has no explanation of what i32 actually means. I have added an explanation for the first time the reader encounters this type.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pcwalton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[steveklabnik]

@bors: r+ 74ba529 rollup

[steveklabnik]

Thanks!