Make HashSet::Insert documentation more consistent

[bwinterton]

I have made the HashSet::Insert documentation more consistent in the use of the term 'value' vs 'key'. Also clarified that if this value is present true is returned, instead of the ambiguous 'a value present'.

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.

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[mitaa]

Could you also update / make this consistent with BTreeSet::insert?

[steveklabnik]

@bors: r+ rollup

Thank you! As @mitaa says, it would be nice to fix BTreeSet as well, feel free to send in a PR if you have the time.

[bors]

📌 Commit 3e9ea3b has been approved by steveklabnik