os: document that Interrupt might not work on every os
os: document that Interrupt might not work on every os

Fixes issue 6720.

LGTM=bradfitz
R=golang-codereviews, iant, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/92340043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang