On the diff man and INFO pages there is no mention if diff has any GNU extensions in it, or portability gotchas, so maybe mention it is POSIX compatible or something. One worries there are extensions but just not mentioned. E.g., "I am about to use diff -U0. But I have an uneasy feeling that it might not work on a non-GNU system but I am offline so can't do an exhaustive check, and the docs don't mention anything confirming that they remembered to mention what options are GNU extensions... Not even one."
