Fix panic message for BTreeSet's range API and document panic cases

[tnballo]

Currently, the panic cases for BTreeSet's range API are undocumented and produce a slightly wrong panic message (says BTreeMap instead of BTreeSet).

Panic case 1 code:

use std::collections::BTreeSet;
use std::ops::Bound::Excluded;

fn main() {
    let mut set = BTreeSet::new();
    set.insert(3);
    set.insert(5);
    set.insert(8);

    for &elem in set.range((Excluded(&3), Excluded(&3))) {
        println!("{elem}");
    }
}

Panic case 1 message:

thread 'main' panicked at 'range start and end are equal and excluded in BTreeMap', /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/alloc/src/collections/btree/search.rs:105:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Panic case 2 code:

use std::collections::BTreeSet;
use std::ops::Bound::Included;

fn main() {
    let mut set = BTreeSet::new();
    set.insert(3);
    set.insert(5);
    set.insert(8);

    for &elem in set.range((Included(&8), Included(&3))) {
        println!("{elem}");
    }
}

Panic case 2:

thread 'main' panicked at 'range start is greater than range end in BTreeMap', /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/alloc/src/collections/btree/search.rs:110:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This PR fixes the output messages to say BTreeSet, adds the relevant unit tests, and updates the documentation for the API.

[rust-highfive]

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• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[rust-highfive]

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

Please see the contribution instructions for more information.

[thomcc]

I'd like to see the changes mentioned in #98039 (comment).

@rustbot label +S-waiting-on-author -S-waiting-on-review

[thomcc]

Can you rebase rather than merge? Or squash the commits? I think we try to avoid merge commits in the history. Other than that this looks great.

[thomcc]

Looks great, thanks for working through this!

@bors r+

[bors]

📌 Commit 774e814 has been approved by thomcc

[tnballo]

Sure, rebased and squashed.