[SE-0285] Amend API design guidelines (#1163)
[SE-0285] Amend API design guidelines (#1163)

Modifies proposal to include a new API design guideline describing when `#fileID`, `#filePath`, or `#file` should be used.
abertelrud/swift-evolution,apple/swift-evolution,natecook1000/swift-evolution,apple/swift-evolution,natecook1000/swift-evolution,abertelrud/swift-evolution