net: Resolver.LookupIP: document should clarify the return values

[yegle]

The current document on net.Resolver.LookupIP was not clear on the return values (i.e. what to expect when err is nil).

Looking into the source code, LookupIP uses internetAddrList here, and the comment for internetAddrList here does state the slice would contain at least one IP when err is nil.

We should duplicate the comment of internetAddrList to LookupIP.

[gopherbot]

Change https://golang.org/cl/281552 mentions this issue: net: clarify Resolver.LookupIP return values

[ianlancetaylor]

Why is this specific to Resolver.LookupIP? Seems like exactly the same issue arises for LookupHost, Resolver.LookupHost, LookupIP, Resolver.LookupIPAddr, LookupSRV, Resolver.LookupSRV, LookupMX, Resolver.LookupMX, LookupNS, Resolver.LookupNS, LookupTXT, Resolver.LookupTXT, LookupAddr, Resolver.LookupAddr, and possibly others.

[yegle]

It might apply to other functions as well, I only encountered this problem when using LookupIP.

[ianlancetaylor]

My point is: let's try to find a clean fix for all relevant functions and methods, rather than just fixing one location. Thanks.

[gopherbot]

Change https://golang.org/cl/314989 mentions this issue: net: clarify return values from Lookup* calls