Adding documentation to clarify the result of DispatchTime(uptimeNanoseconds: 0)
Adding documentation to clarify the result of DispatchTime(uptimeNanoseconds: 0)

(Radar 28814085)
(SR-2807)

Signed-off-by: Daniel A. Steffen <bc823475b60dbcbd7be1bbdfe095b5f1939d5bd2@apple.com>

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch