[stdlib] Document stable sort guarantee
[stdlib] Document stable sort guarantee

Changes the documentation of the stdlib `sort`/`sorted` methods
to provide a stability guarantee. No changes to behavior.

roambotics/swift,apple/swift,apple/swift,apple/swift,roambotics/swift,apple/swift,roambotics/swift,apple/swift,apple/swift,roambotics/swift,roambotics/swift,roambotics/swift