Clarify port semantics WRT test/run-pass/task-comm-16.rs

[catamorphism]

Answer the question in the FIXME: "Do port semantics really guarantee these [operations] happen in order?"

[eholk]

I hope the answer is yes, although I don't know who gets to make the official determination.

[catamorphism]

Since the test passes, it seems like the answer should indeed be yes... but then, we all know the difference between semantics and implementation :-)