[OrderedCollections] Update docs to state that the sort algorithm is stable

[lorentey]

Resolves #186.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[lorentey]

@swift-ci test

[benrimmington]

Should the three sort(by:) comments still refer to the areInIncreasingOrder parameter?

https://github.com/apple/swift/blob/5d832bf946bef47a1a7e894ca32b9768799f1863/stdlib/public/core/Sort.swift#L234-L236

[lorentey]

Oops, you're absolutely right, I mistakenly lost the custom wording for the closure-taking variants. Restored!

[lorentey]

@swift-ci test