Improve documentation for `subset_of()`

[markozagar]

I was working through the examples in rust-advanced-testing-workshop/collections, and got stuck at this exercise:

#[googletest::gtest]
fn failed_subset_of() {
    let x = btreeset![1, 2, 3];
    let y = btreeset![3, 4];
    assert!(y.is_subset(&x));
    }

I gave it a solid 30 minutes of trying in various ways, but I couldn't figure out how to write this particular test. This is the closest I'd gotten:

assert_that!(y, subset_of(x.iter().collect::<std::collections::BTreeSet<&i32>>()));

I don't think this is a good way to do this, but I can't figure out how to massage x into an acceptable form.

It would be helpful if the examples in the docs for subset_of() showed usage from variables (of different collection types), rather than just arrays. The test module in subset_of.rs is similarly unhelpful. There isn't a large body of code using googletest-rust, so searching online also led nowhere.

[markozagar]

Well, apparently the canonical solution from rust-advanced-testing-workshop is:

   let x = btreeset![&1, &2, &3];
   let y = btreeset![3, 4];
   assert_that!(y, subset_of(x));

I'm not sure I like it though, changing input data to fit the test feels icky.

That makes usage of subset_of() straightforward, so you can probably close this, but it might still be a good idea to provide at least one or two non-array examples in the docs...