Clarify the docs for `partitioned(by:)`

[j-f1]

As currently written, the summary for partitioned(by:) heavily implies the return value is (trueValues, falseValues). In fact, it’s the opposite. I rephrased this summary to clarify that (1) the order is (falseValues, trueValues) and (2) the elements of the two returned arrays are in the order they were in the original sequence/collection.

Checklist

• (N/A) I've added at least one test that validates that my change is working, if appropriate
• [x] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [x] I've updated the documentation if necessary

[j-f1]

@swift-ci please test

[j-f1]

@swift-ci please test

[j-f1]

@swift-ci please test