Improve information about loops

[withoutboats]

1. Moved 'while' section below 'loop', 'break', and 'continue'; 2) Added information to 'while' and 'for' loops that they interact with 'break' and 'continue' and may have a lifetime label. 3) Clarified labeling syntax on the infinite loops.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (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 The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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]

Why move while down? Just curious.

[alexcrichton]

r? @steveklabnik

[withoutboats]

It was so it would not be antecedent to the information about break and continue (which it now references), and because (like for) it builds on the basic concept of loop.

[steveklabnik]

Seems like a good reason. Thanks!

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit 3ae6a5e has been approved by steveklabnik

[Diggsey]

The grammar for "while let" loops is missing the lifetime label and associated comment that break/continue work with it. (I've tested and rust does allow labels and break/continue here)