Update BitSet docs with correct types

[DrKwint]

The functions BitSet::{iter,union,symmetric_difference} each had docs that claimed u32s were output when their actual output each end up being usizes.

r? @steveklabnik

[rust-highfive]

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 The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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

Thank you for this PR! there's three things:

1. there's an extra file in here for some reason
2. when you fix that, can you squash as well
3. I'm good to merge this afterward, which means you should comment over at 1.0 release notes and authors updates #25196 so you can get in AUTHORS

[DrKwint]

Sorry, I'm not terribly experienced with Git. I think I've fixed it though. r? @steveklabnik

[alexcrichton]

@bors: r+ c98df58 rollup

Thanks!