net: document that SplitHostPort and JoinHostPort never treat non-IP literals enclosed in square brackets

[mikioh]

As per #18059, we need to document that both SplitHostPort and JoinHostPort never treat non-IP literals enclosed in square brackets, because the output of JoinHostPort must be able to be used for connection setup functions and methods; net.Dial("tcp", "[name%zone]:8080") is not defined behavior and hard to solve the ambiguity of "name%zone".

[gopherbot]

CL https://golang.org/cl/40510 mentions this issue.